Chronic interstitial nephritis is a progressive tubulointerstitial kidney disease characterized by chronic inflammation and fibrosis of the renal interstitium, leading to tubular atrophy and gradual decline in renal function.

Exam domain	One-glance essentials
Definition / diagnosis	*Chronic interstitial inflammation with tubular atrophy and interstitial fibrosis causes progressive CKD* ^[1]^[2] Tubular dysfunction is often disproportionate to glomerular findings ^[2]
Epidemiology / risks	NSAIDs/analgesics, PPIs, lithium and calcineurin inhibitors are major drug causes ^[3] *Aristolochic acid exposure causes CIN plus urothelial malignancy risk*; also consider obstruction/reflux, metabolic, autoimmune, TB and inherited disease ^[3]^[4]
Core mechanism	Recurrent tubular injury → inflammation → TGF-β/fibroblast activation → fibrosis and tubular atrophy ^[2] Peritubular capillary loss → chronic hypoxia → further fibrosis, creating a self-amplifying cycle ^[2]
Clinical picture	Insidious CKD with polyuria/nocturia, salt wasting, disproportionate anaemia and electrolyte/acid-base disturbance ^[2] Urinary sediment is less nephritic than glomerular disease; ask directly about OTC analgesics, PPIs, lithium and TCM ^[3]
Investigations	*Sterile pyuria, WBC casts and tubular proteinuria usually < 1–1.5 g/day* favour CIN ^[2] RFT/electrolytes may show NAGMA/RTA; ultrasound often shows small echogenic kidneys ^[2] Biopsy shows interstitial fibrosis + tubular atrophy + chronic inflammatory infiltrate when diagnosis/actionability remains uncertain ^[2]
Management	*Stop the culprit or treat obstruction, infection, metabolic or autoimmune cause early* ^[3] Established fibrosis receives supportive CKD care: BP/proteinuria management, alkali/electrolytes, anaemia/bone care and RRT planning ^[2] Steroids help only a selected active inflammatory component, not fixed fibrosis ^[3]
Complications	Progressive CKD/ESKD, anaemia, acidosis, electrolyte disturbance and CKD-mineral bone disease ^[2] Analgesic nephropathy may cause papillary necrosis; aristolochic acid adds upper-tract urothelial carcinoma risk ^[3]^[4]
Prevention / follow-up	Avoid chronic unnecessary NSAID/PPI exposure; monitor renal function and urine concentrating defects during lithium/CNI therapy ^[3] Prevent recurrent pyelonephritis/reflux injury and relieve chronic obstruction promptly ^[2]
Exam traps	*In Hong Kong, ask specifically about TCM, OTC NSAIDs and long-term PPIs* ^[3]^[4] Heavy albuminuria, dysmorphic RBCs or RBC casts suggest glomerular disease, not isolated CIN ^[2] Do not biopsy very small scarred kidneys when histology is unlikely to change management ^[2]

References

^[1] Lecture slides: GC 057. Glomerular and Tubulo-interstitial Diseases and Acute Kidney Injury.pdf (p.59) ^[2] Senior notes: Block A - Glomerular and Tubulo-interstitial Diseases and Acute Kidney Injury.pdf (p.28–29); Ryan Ho Urogenital.pdf (Section 4.2 – Chronic Interstitial Nephritis) ^[3] Lecture slides: GC 043. Drugs and the Kidney.pdf (p.16); Senior notes: Block A - Chronic Kidney Disease and its Complications.pdf (p.6–12) ^[4] Senior notes: Block A - Drugs and the Kidney.pdf (p.16 – Aristolochic acid nephropathy)

Chronic Interstitial Nephritis (CIN)

Chronic interstitial nephritis (CIN) is a clinicopathological entity characterised by generalized chronic inflammatory infiltrate of the interstitium with tubular atrophy and generalized interstitial oedema or fibrosis ^[1]^[2]. Breaking down the name:

"Chronic" → insidious, progressive process over weeks to months (as opposed to acute interstitial nephritis which develops over days)
"Interstitial" → affecting the renal interstitium — the connective tissue space between the tubules
"Nephritis" → "nephros" (kidney) + "itis" (inflammation)

CIN is the major pathway leading to chronic kidney disease ^[3]. Unlike glomerular diseases that primarily damage the filtration barrier, CIN targets the tubulointerstitial compartment — which constitutes 95% of the kidney's volume ^[3]. This is why any pathology here has devastating consequences for overall renal function.

Core Concept

Think of it this way: the glomerulus is the filter, but the tubules and interstitium are the "processing plant" that fine-tunes the filtrate. If the processing plant is chronically inflamed and scarred, you lose concentrating ability, electrolyte handling, and acid-base regulation — often before you lose filtration capacity.

Chronic TIN is an important cause of chronic kidney disease ^[1]. It represents the "final common pathway" of many different insults — whether drugs, toxins, metabolic derangements, infections, or autoimmune diseases — that all converge on interstitial fibrosis and tubular atrophy (IF/TA), the histological hallmark of irreversible renal damage.

2. Epidemiology

CIN accounts for approximately 15–30% of cases of CKD in different series worldwide
In Hong Kong specifically, tubulointerstitial diseases (including CIN) account for ~5% of paediatric CKD ^[4], but in the adult population the proportion is higher when drug-induced causes (NSAIDs, traditional Chinese medicine [TCM], calcineurin inhibitors) are included
The first and second most common causes of CKD in Hong Kong are diabetes (~51%) and hypertension/vascular causes ^[5], but CIN is an important "hidden" cause — often missed because it presents insidiously without nephrotic-range proteinuria or dramatic haematuria

No strong sex predilection overall, though specific aetiologies have demographic patterns:
- Analgesic nephropathy: historically more common in middle-aged women (heavy analgesic users)
- Aristolochic acid nephropathy: historically significant in Hong Kong and mainland China, now declining since the ban in 2004
- Reflux nephropathy: typically presents in young adults who had childhood vesicoureteral reflux
CIN can affect any age group, but clinical presentation is typically in adult life ^[1]

Risk Factor	Mechanism	Hong Kong Relevance
Chronic NSAID/analgesic use	Direct tubulotoxicity + papillary ischaemia → papillary necrosis + CIN	Very common; NSAIDs often OTC
TCM containing aristolochic acid	Direct tubulotoxicity + urothelial carcinogenesis	Banned since 2004 ^[6], but historical cases still seen
Proton pump inhibitor (PPI) use	Immune-mediated AIN → if untreated → CIN	PPIs known to cause chronic TIN → CKD ^[5]
Chronic lithium use	Downregulates aquaporin-2 → concentrating defect + chronic tubulointerstitial fibrosis	Psychiatric patients on long-term lithium
Calcineurin inhibitor use (ciclosporin, tacrolimus)	Chronic arteriolar vasoconstriction → ischaemic TIN + direct tubulotoxicity	Post-transplant patients
Metabolic: hypercalcaemia, hypokalaemia, hyperuricaemia	Nephrocalcinosis, direct tubular injury
Chronic urinary tract obstruction / reflux	Back-pressure → tubulointerstitial inflammation and fibrosis	BPH in elderly men; reflux in children
Heavy metal exposure (lead, cadmium)	Proximal tubular toxicity → chronic interstitial fibrosis	Occupational exposure
Autoimmune diseases (Sjögren's, SLE, sarcoidosis)	Lymphocytic infiltration of interstitium

3. Anatomy and Function of the Tubulointerstitium

To understand CIN, you must understand what the tubulointerstitium does:

Structure	Key Functions
Proximal tubule	Reabsorbs ~65% filtered Na⁺, glucose, amino acids, phosphate, bicarbonate; secretes organic acids, drugs, toxins
Loop of Henle	Creates the medullary concentration gradient (countercurrent multiplication); reabsorbs ~25% filtered Na⁺
Distal tubule	Fine-tunes Na⁺/K⁺ balance; Ca²⁺ reabsorption (PTH-dependent)
Collecting duct	ADH-responsive water reabsorption (aquaporin-2); H⁺/K⁺ secretion; final urine concentration
Interstitium	Scaffolding support for tubular architecture; participates in fluid and electrolyte exchange; endocrine functions ^[7] — including EPO production by peritubular interstitial fibroblasts

Why CIN Causes Disproportionate Anaemia

EPO is produced by peritubular interstitial fibroblasts in the renal cortex. In CIN, the interstitium is the primary site of damage → fibroblasts are replaced by scar tissue → prominent anaemia that may be more severe than expected for the degree of GFR decline ^[1]. This contrasts with glomerular diseases where anaemia correlates more predictably with GFR.

The renal medulla is inherently vulnerable to injury because:

Relatively hypoxic: The vasa recta operate as countercurrent exchangers, creating a low-oxygen environment in the inner medulla (pO₂ as low as 10–20 mmHg)
High metabolic demand: The thick ascending limb of the Loop of Henle actively pumps Na⁺/K⁺/2Cl⁻ and consumes large amounts of ATP
Concentration of toxins: The countercurrent system concentrates drugs and toxins in the medullary interstitium

This is why many causes of CIN preferentially damage the medulla first, leading to concentrating defects (polyuria, nocturia) as an early clinical feature before GFR drops significantly.

4. Aetiology (with Hong Kong Focus) and Pathophysiology

The four clinical entities within "tubulointerstitial nephritis" are: (1) Acute interstitial nephritis, (2) Chronic interstitial nephritis, (3) K⁺-wasting tubular disorders, and (4) Renal tubular acidosis ^[3]^[8].

CIN specifically arises from multiple aetiological categories. I will organise these systematically and explain the pathophysiology for each.

Mechanism: AIN that is not recognised and treated (e.g., offending drug not withdrawn) → persistent inflammation → progressive fibrosis → CIN

This is perhaps the most important conceptual point: any cause of AIN, if not resolved, can progress to CIN
The transition from AIN to CIN involves:
1. Ongoing inflammatory infiltrate (T-lymphocytes, macrophages)
2. Release of profibrotic cytokines (TGF-β, PDGF, CTGF)
3. Fibroblast activation and collagen deposition
4. Tubular epithelial-to-mesenchymal transition (EMT)
5. Replacement of normal architecture with fibrotic tissue

Clinical Pearl

The most common mistake students make: assuming AIN always resolves. If you don't remove the offending drug or treat the underlying cause, AIN → CIN → CKD. PPIs are a classic example: they cause acute TIN → and if unrecognised, chronic TIN → CKD ^[5].

4.2 Drug-Induced CIN

This is one of the most important aetiological categories in clinical practice.

Drugs implicated: analgesics (NSAIDs, paracetamol) ^[2]

Pathophysiology:

Characterised by papillary necrosis and chronic interstitial nephritis ^[1]
NSAIDs inhibit cyclooxygenase (COX) → reduced prostaglandin synthesis → loss of prostaglandin-mediated vasodilation of the vasa recta → medullary ischaemia → papillary necrosis
Paracetamol (when used chronically, especially in combination with aspirin/caffeine): oxidative metabolites accumulate in the medulla → direct tubular toxicity + lipid peroxidation
The combination is worse than either alone (historically "phenacetin nephropathy" — phenacetin is metabolised to paracetamol)
Previously an important cause of CKD, nowadays much less common ^[1] due to withdrawal of phenacetin and awareness campaigns

NSAIDs can cause: vasoconstriction, AKI, nephrotic syndrome, papillary necrosis, and tubulointerstitial nephritis ^[5]^[9]

Calcineurin inhibitors are known to cause vascular calcification, AKI, and CKD ^[5]

Pathophysiology:

Chronic afferent arteriolar vasoconstriction → chronic medullary ischaemia
Direct tubular toxicity
Promotion of TGF-β → interstitial fibrosis
"Striped fibrosis" is the characteristic histological pattern — bands of fibrosis radiating from the medulla into the cortex
Important in renal transplant patients — chronic allograft injury features interstitial fibrosis and tubular atrophy ^[10]

Pathophysiology:

Lithium enters collecting duct principal cells via the epithelial sodium channel (ENaC)
Inhibits glycogen synthase kinase-3β (GSK-3β) → decreasing expression of aquaporin-2 (AQP2) ^[11] → nephrogenic diabetes insipidus → polyuria
Chronic lithium use → interstitial fibrosis + microcyst formation + tubular atrophy
Unique histological feature: microcystic tubular dilatation
Can persist even after lithium discontinuation

Cisplatin is known to cause tubular diseases, resulting in hypokalaemia and hypomagnesaemia ^[5]

Pathophysiology:

Concentrated in the proximal tubular cells (via OCT2 transporter) → mitochondrial injury → apoptosis
Damages all tubular segments → can cause Fanconi-like syndrome
Chronic exposure → interstitial fibrosis

Tenofovir (nucleotide reverse transcriptase inhibitor for HIV/HBV) → proximal tubular toxicity → Fanconi syndrome → if prolonged → CIN

PD-1 inhibitors are known to cause acute tubulointerstitial nephritis → AKI ^[5]

Pathophysiology: Immune checkpoint blockade → loss of peripheral T-cell tolerance → autoreactive T-cells attack renal tubular antigens → AIN → can progress to CIN if unrecognised

4.3 Toxin-Induced CIN

Aristolochic acid is a nephrotoxic substance found in some Chinese herbal medications and some grains, also in Eastern Europe ^[6]

Known to cause: (1) Chronic interstitial nephritis and CKD, (2) Multi-focal uro-epithelial malignancies ^[6]

Pathophysiology:

Aristolochic acid (AA) → metabolised to aristolactam → forms DNA adducts in tubular epithelial cells AND urothelial cells
Tubular cells: apoptosis → atrophy → interstitial fibrosis (relatively paucicellular — i.e., fibrosis WITHOUT much inflammation)
Urothelial cells: DNA adducts → TP53 mutations → urothelial carcinoma (bladder, ureter, renal pelvis)
Banned in Hong Kong since 2004 ^[6]
Some patients had to undergo prophylactic removal of the urinary tract to prevent malignancy ^[6]
Historically known as "Balkan endemic nephropathy" in Eastern Europe (exposure through contaminated grain)

High Yield: Aristolochic Acid in Hong Kong Context

This is a classic HKUMed exam topic. Remember the dual pathology: CIN + urothelial malignancy. Even though it's now banned, patients with historical exposure still present with late-onset CKD or urothelial cancers.

Pathophysiology:

Chronic low-level lead exposure → accumulates in proximal tubular cells → mitochondrial dysfunction + nuclear inclusion bodies
Leads to: proximal tubular dysfunction (Fanconi-like) → later, chronic interstitial fibrosis
Associated with: gout (lead inhibits urate secretion → "saturnine gout"), hypertension
May be seen in occupational exposure (battery workers, painters, moonshine drinkers)

Cadmium binds to metallothionein → accumulates in proximal tubules → tubular proteinuria, Fanconi syndrome → chronic interstitial fibrosis
Industrial/environmental exposure (Itai-itai disease in Japan)

4.4 Metabolic Causes

Pathophysiology:

Chronic hypercalcaemia → calcium deposition in the renal medullary interstitium and tubular basement membranes
Direct tubular toxicity + obstruction of tubular lumina by calcium deposits
Also impairs concentrating ability (calcium antagonises ADH action on collecting duct)
Results in: polyuria (nephrogenic DI), distal RTA, CIN

Pathophysiology:

Severe chronic hypokalaemia → "hypokalaemic nephropathy"
Mechanism: intracellular K⁺ depletion → tubular cell vacuolisation (especially proximal tubule and medullary collecting duct) → impaired concentrating ability + interstitial fibrosis
Hypokalaemia can itself perpetuate metabolic disturbances through tubular dysfunction ^[12]
Also stimulates ammonia production → complement activation in the interstitium → inflammation

Pathophysiology:

Chronic hyperuricaemia → urate crystal deposition in the medullary interstitium → foreign-body giant cell reaction → chronic inflammation → fibrosis
Distinct from acute uric acid nephropathy (intratubular crystal obstruction, e.g., tumour lysis syndrome)

Primary (genetic) or secondary (enteric, dietary) → calcium oxalate crystal deposition in tubules and interstitium → chronic inflammation → fibrosis

4.5 Structural / Obstructive Causes

Reflux nephropathy is listed under urinary tract obstruction as a cause of CIN ^[2]

Pathophysiology:

Vesicoureteral reflux (VUR) → infected urine refluxes into the renal parenchyma → recurrent episodes of pyelonephritis
Chronic inflammation → interstitial scarring, typically in the poles of the kidney (where compound papillae allow intrarenal reflux)
Leads to: focal scarring, cortical thinning, calyceal blunting
May present in adulthood with CKD, hypertension, proteinuria
Chronic pyelonephritis is listed as a cause of secondary hypertension ^[13]

Chronic obstruction → dilatation of the system → tubular dysfunction ^[11]

Pathophysiology:

Back-pressure → tubular dilatation → ischaemia → inflammatory infiltrate → fibrosis
Prolonged obstruction will progress to tubulointerstitial fibrosis ^[14]

Condition	Inheritance	Key Features
Nephronophthisis	Autosomal recessive	Corticomedullary cysts, polyuria, anaemia; most common genetic cause of ESRD in children/young adults
Medullary cystic kidney disease	Autosomal dominant	Similar to nephronophthisis but later onset; now reclassified as "autosomal dominant tubulointerstitial kidney disease" (ADTKD)
Cystinosis	Autosomal recessive	Lysosomal cystine accumulation → proximal tubular dysfunction (Fanconi syndrome) → progressive CIN
Primary hyperoxaluria	Autosomal recessive	Oxalate overproduction → nephrocalcinosis → CIN
Methylmalonic acidaemia	Autosomal recessive	Organic acid accumulation → tubular toxicity
Sickle cell disease	Autosomal recessive	Sickling in vasa recta → medullary ischaemia → papillary necrosis → CIN

4.7 Autoimmune / Systemic Disease

Sjögren's syndrome can cause renal manifestations including RTA and GN ^[15]

Chronic interstitial nephritis with positive rheumatoid factor and positive anti-Ro → Sjögren syndrome ^[11]

Pathophysiology:

Lymphocytic infiltration of the renal interstitium (analogous to lymphocytic infiltration of salivary/lacrimal glands)
Predominantly affects distal tubule and collecting duct → distal RTA (Type 1)
Can present as: hypokalaemia, metabolic acidosis, nephrogenic DI, nephrocalcinosis
Graves' disease can be associated with autoimmune interstitial nephritis, especially in the context of Sjögren's syndrome ^[11]

Pathophysiology:

Non-caseating granulomata in the renal interstitium
Also causes hypercalcaemia (via 1α-hydroxylase activity in granulomata) → nephrocalcinosis → additional tubular injury
Double-hit: granulomatous interstitial nephritis + metabolic (calcium) injury

SLE is well-known for glomerular involvement (lupus nephritis), but ~60-70% of lupus nephritis biopsies also show tubulointerstitial inflammation
Isolated TIN in SLE is rare but documented
Immune complex deposition along tubular basement membranes

Part of the IgG4-related disease spectrum → dense lymphoplasmacytic infiltrate with IgG4+ plasma cells + storiform fibrosis
Increasingly recognised cause of CIN
Responds to corticosteroids

Inflammation: TB, GPA, sarcoidosis ^[2]

Granulomatous interstitial nephritis + small vessel vasculitis
ANCA-associated

Infection	Mechanism
Tuberculosis ^[13]	Granulomatous interstitial nephritis; can cause papillary necrosis, ureteral strictures
Chronic bacterial pyelonephritis	Recurrent infection → scarring → CIN (see reflux nephropathy)
BK polyomavirus (post-transplant)	Viral replication in tubular cells → tubulointerstitial damage → graft loss
HIV-associated nephropathy (HIVAN)	Collapsing FSGS is more classic, but tubulointerstitial inflammation is also a feature

Radiation ^[2] involving the kidneys → endothelial injury → chronic vascular damage → ischaemic tubulointerstitial fibrosis. Typically occurs 6–12 months after radiation therapy involving the renal field.

In a significant proportion of CIN cases, no specific cause is identified
Chronic interstitial nephritis — etiology not well known ^[3] (as stated on GC slides)

5. Classification

CIN can be classified by several axes:

5.1 By Aetiology (as above)

Pattern	Characteristics	Examples
Lymphocytic/mononuclear infiltrate	T-cells, macrophages, plasma cells	Drug-induced, autoimmune (Sjögren's, SLE)
Granulomatous	Non-caseating or caseating granulomata	Sarcoidosis, TB, GPA, drug-induced
Fibrotic (paucicellular)	Minimal inflammatory cells, predominant fibrosis	Aristolochic acid nephropathy, chronic obstruction, late-stage any cause
Crystal-associated	Crystal deposition with foreign-body reaction	Uric acid, calcium oxalate, calcium phosphate

Site	Clinical Consequence	Example
Medullary predominant	Concentrating defect (polyuria/nocturia), papillary necrosis	Analgesic nephropathy, sickle cell
Cortical predominant	Loss of GFR, uraemia	Late-stage CIN, calcineurin inhibitor toxicity
Diffuse	Combined features	Most advanced CIN

Drug-induced kidney disease can be classified by time: Acute ( < 7 days), Subacute (7–90 days), Chronic ( > 90 days) ^[5]

6. Clinical Features

The clinical features of CIN reflect the underlying tubulointerstitial damage. The key principle: tubular dysfunction manifests BEFORE significant glomerular dysfunction. This contrasts with primary glomerular diseases where proteinuria and haematuria dominate early.

Symptom	Pathophysiological Basis
Nocturia and polyuria (EARLY)	Tubular damage to the medullary area → loss of medullary concentration gradient → inability to concentrate urine in response to ADH ^[1]. This is the earliest and most characteristic symptom. The countercurrent multiplication system is disrupted, so the kidney cannot produce concentrated urine at night → nocturia
Polydipsia / thirst	Compensatory response to polyuria and water loss; also driven by acidosis + hyperventilation ^[5]
Fatigue / malaise	Multifactorial: anaemia (disproportionately severe for degree of GFR decline) ^[1] + uraemia + metabolic acidosis + electrolyte disturbance
Nausea / poor appetite / weight loss	Uraemic symptoms — accumulation of nitrogenous waste products that are normally excreted by tubular secretion
Itch (uremic pruritus)	Accumulation of uraemic toxins + secondary hyperparathyroidism + altered nociception; occurs in advanced CKD
Muscle weakness / cramps	Electrolyte disturbance (hypokalaemia, hypocalcaemia, hypermagnesaemia) + metabolic acidosis
Bone pain (late)	CKD-MBD (mineral and bone disorder): secondary hyperparathyroidism → osteitis fibrosa cystica; or adynamic bone disease
History of causative exposure	Drug history (NSAIDs, PPIs, lithium, TCM), occupational exposure (lead, cadmium), family history (genetic causes), autoimmune symptoms (dry eyes/mouth in Sjögren's)

Key Symptom Pattern

CKD from CIN is often insidious: "Many patients will come to clinic with a creatinine of 200 to 300 without even knowing that they have a kidney problem" ^[5]. The earliest clue is often nocturia or mild polyuria — easily dismissed by patients.

Sign	Pathophysiological Basis
Low/normal blood pressure (EARLY)	Salt wasting from tubular damage → hypovolaemia → lower BP ^[1]. This is a distinguishing feature from glomerular CKD, where hypertension is early and prominent. CIN-related hypertension is unusual until late ^[1] when cortical damage and loss of nephron mass predominate
Dehydration signs	Dry mucous membranes, reduced skin turgor, sunken eyes — reflect polyuria + salt wasting
Pallor	Prominent anaemia (NcNc) — disproportionate to GFR decline ^[1] because EPO-producing interstitial fibroblasts are directly damaged by the inflammatory/fibrotic process
Café au lait complexion (late)	Combination of anaemia pallor + retained uraemic pigments (urochromes) in advanced CKD
Absence of significant oedema (early)	Unlike nephrotic syndrome/glomerular disease, CIN does not cause heavy proteinuria → no severe hypoalbuminaemia → no dramatic oedema
Bilateral palpably small kidneys (late)	Progressive fibrosis → cortical thinning → kidney shrinkage. On ultrasound: bilateral small kidneys with increased echogenicity and loss of corticomedullary differentiation
Signs of underlying cause	Dry eyes/mouth (Sjögren's), skin rash (drug reaction), gouty tophi (urate nephropathy), features of sarcoidosis (erythema nodosum, bilateral hilar lymphadenopathy), lead lines on gums
Signs of uraemia (late)	Pericardial friction rub (uraemic pericarditis), asterixis, scratch marks, peripheral neuropathy

This is critical for understanding what makes CIN different from glomerular CKD:

Feature	CIN Pattern	Glomerular CKD Pattern	Why?
Proteinuria	Mild (usually < 1–1.5 g/day); tubular proteinuria (low MW proteins: β2-microglobulin, α1-microglobulin)	Heavy (often > 3.5 g/day); albumin predominant	In CIN, the glomerular filtration barrier is relatively intact → albumin not lost in large quantities. But damaged tubules cannot reabsorb the small proteins normally filtered
Haematuria	Absent or mild; no dysmorphic RBCs or RBC casts	Often present with dysmorphic RBCs and RBC casts	No glomerular basement membrane damage in CIN
Urine sediment	Sterile pyuria (WBCs without bacteria) ^[14]; WBC casts; sometimes eosinophiluria (if residual allergic component)	RBC casts, dysmorphic RBCs	Interstitial inflammation → WBC migration into tubular lumen
Anaemia	Disproportionately severe for GFR ^[1]	Proportional to GFR	Direct destruction of EPO-producing interstitial fibroblasts
Electrolytes	Pronounced disturbance: hyperK⁺, metabolic acidosis (RTA), sometimes hypokalaemia (if proximal tubular dysfunction or specific causes like Sjögren's) ^[1]	Electrolyte disturbance proportional to GFR decline	Direct tubular transport defects
Phosphate	May be NORMAL (unlike other CKD causes where phosphate is elevated) ^[1]	Elevated (hyperphosphataemia)	Damaged proximal tubules cannot reabsorb phosphate → phosphaturia → normal serum phosphate despite reduced GFR
Blood pressure	Normal or low (early); hypertension unusual until late	Early hypertension	Salt wasting from tubular damage
Kidney size	Small with preserved shape (unless obstruction)	Variable (enlarged in DM, APCKD; small in chronic GN)	Fibrosis causes uniform shrinkage

High Yield Exam Point

The "fingerprint" of CIN vs glomerular CKD: tubular proteinuria ( < 1.5g/day), sterile pyuria, disproportionate anaemia, pronounced electrolyte/acid-base disturbance, normal phosphate, and relatively preserved BP until late. If you see these features in an MCQ or clinical scenario, think CIN.

Depending on which part of the tubule is predominantly damaged, CIN can present with specific tubular dysfunction patterns:

Tubular Segment Damaged	Resulting Syndrome	Clinical Features
Proximal tubule	Fanconi syndrome	Glucosuria (without hyperglycaemia), aminoaciduria, phosphaturia, bicarbonaturia (Type 2 RTA), uricosuria, low MW proteinuria
Thick ascending limb	Bartter-like picture	Hypokalaemia, metabolic alkalosis, hypercalciuria
Distal tubule	Distal RTA (Type 1)	NAGMA, hypokalaemia, inability to acidify urine (pH > 5.5), nephrocalcinosis
Collecting duct	Nephrogenic DI	Polyuria, polydipsia, hypernatraemia (if water access limited)
Collecting duct (principal cells)	Type 4 RTA / hyperK⁺	NAGMA with hyperkalaemia, hyperchloraemia — due to aldosterone resistance or deficiency

Regardless of the initial insult, CIN converges on a common pathological sequence:

Key points:

Tubular injury is the trigger — whether from direct toxicity, ischaemia, immune attack, or crystal deposition
Injured tubular cells release damage-associated molecular patterns (DAMPs) and pro-inflammatory mediators
This recruits inflammatory cells (T-lymphocytes, macrophages)
Inflammatory cells release profibrotic cytokines (especially TGF-β)
Myofibroblast activation (from resident fibroblasts, pericytes, or via EMT of tubular cells) → collagen deposition
Peritubular capillary loss (rarefaction) → chronic hypoxia → MORE fibrosis (a self-perpetuating vicious cycle)
The end result is interstitial fibrosis and tubular atrophy (IF/TA) — the histological hallmark that is IRREVERSIBLE

The Rarefaction-Hypoxia-Fibrosis Cycle

This vicious cycle is why CKD from CIN is progressive even after the initial insult is removed. Once you lose peritubular capillaries, the resulting chronic hypoxia drives ongoing fibrosis independent of the original cause. This is the fundamental reason why early detection and intervention are critical.

High Yield Summary

Chronic Interstitial Nephritis — Key Points for Exams:

CIN = chronic inflammatory infiltrate + tubular atrophy + interstitial fibrosis; constitutes 95% of kidney volume affected ^[3]
Major pathway leading to CKD ^[3]; an important but often underrecognised cause
Top aetiologies (HK context): drugs (NSAIDs, PPIs, lithium, calcineurin inhibitors), aristolochic acid (banned in HK since 2004 — causes CIN + urothelial malignancy) ^[6], metabolic (hypercalcaemia, hypokalaemia, hyperuricaemia), obstruction/reflux, autoimmune (Sjögren's, sarcoidosis), infections (TB), inherited (nephronophthisis, cystinosis)
Any untreated AIN can progress to CIN
Clinical "fingerprint" vs glomerular CKD:
- Tubular proteinuria ( < 1.5 g/day), sterile pyuria, no significant haematuria
- Disproportionate anaemia for GFR ^[1]
- Pronounced electrolyte/acid-base disturbance; phosphate may be normal ^[1]
- Early polyuria/nocturia (concentrating defect); late uraemia
- BP normal/low early; hypertension late
Drug-induced CKD classification: Acute ( < 7 days), Subacute (7–90 days), Chronic ( > 90 days) ^[5]
Pathophysiology final common pathway: Tubular injury → inflammation → TGF-β → fibroblast activation → IF/TA → capillary rarefaction → hypoxia → more fibrosis (vicious cycle)
PPIs: acute TIN → chronic TIN → CKD ^[5]; PD-1 inhibitors: acute TIN → AKI ^[5]
NSAIDs: vasoconstriction, AKI, nephrotic syndrome, papillary necrosis, TIN ^[5]

Active Recall - Chronic Interstitial Nephritis

^[1] Senior notes: Ryan Ho Urogenital.pdf (Section 4.2 – Chronic Interstitial Nephritis) ^[2] Senior notes: Maksim Medicine Notes.pdf (p.234 – Tubulointerstitial nephritis) ^[3] Senior notes: Block A - Glomerular and Tubulo-interstitial Diseases and Acute Kidney Injury.pdf (p.29 – Tubulointerstitial Diseases) ^[4] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.333 – CKD causes) ^[5] Senior notes: Block A - Chronic Kidney Disease and its Complications.pdf (p.6–12 – Causes, Drug-induced CKD) ^[6] Senior notes: Block A - Drugs and the Kidney.pdf (p.16 – Aristolochic acid nephropathy) ^[7] Senior notes: Block A - Nephrotology Teaching Clinic RTD.pdf (p.1 – Introduction to TIN) ^[8] Lecture slides: GC 057. Glomerular and Tubulo-interstitial Diseases and Acute Kidney Injury.pdf (p.59 – Entities) ^[9] Senior notes: Block A – Nephrology Data Interpretation.pdf (p.11 – NSAIDs and TIN) ^[10] Senior notes: Block A - Renal Replacement Therapies.pdf (p.40 – Chronic allograft injury) ^[11] Senior notes: Block A - Two cases of polyuria and polydipsia.pdf (p.4–5 – Nephrogenic DI, Sjögren's) ^[12] Senior notes: learning_points_output.txt (Renal Tubular Disorders learning points) ^[13] Senior notes: Block A - High blood pressure_ hypertension.pdf (p.21 – Secondary HT causes) ^[14] Senior notes: Ryan Ho Critical Care.pdf (p.25–27 – AKI workup, sterile pyuria in TIN) ^[15] Senior notes: Ryan Ho Rheumatology.pdf (p.88 – Sjögren's Syndrome)

Differential Diagnosis of Chronic Interstitial Nephritis

When you encounter a patient with features suggestive of CIN — insidious CKD, mild tubular proteinuria, sterile pyuria, disproportionate anaemia, electrolyte/acid-base disturbance, polyuria/nocturia — you need to think along two axes simultaneously:

Is this truly CIN, or is this CKD from another compartment? (i.e., glomerular, vascular, or obstructive disease mimicking CIN)
If it is CIN, what is the underlying aetiology? (i.e., drugs, toxins, metabolic, autoimmune, structural, inherited, or idiopathic)

The first axis is about anatomical compartment diagnosis — which part of the nephron is primarily damaged. The second axis is about aetiological diagnosis within the CIN category. Both are essential because they drive completely different management strategies.

The aetiologies of renal diseases can be categorised into: vascular disease, glomerular disease, tubulointerstitial disease, and obstructive nephropathy ^[9]. This is the fundamental framework for any patient presenting with renal impairment.

When a patient comes with CKD, the differentials for the cause include: diabetes, hypertension/vascular, chronic glomerulonephritis (e.g. IgA GN), chronic pyelonephritis, polycystic kidney disease, drug-induced/TIN/TCM, myeloma/monoclonal gammopathy, vasculitis/SLE/other autoimmune diseases, obstruction/kidney stones, Alport's or other hereditary diseases, and obesity ^[5].

Glomerular diseases have to be distinguished from tubulointerstitial and vascular disease. Tubulointerstitial disease does NOT directly increase protein excretion but nephron loss can induce secondary glomerulosclerosis leading to proteinuria. Vascular disease such as hypertensive nephropathy or thrombotic angiopathy can give rise to proteinuria and haematuria resembling a glomerular disease ^[16].

This is a critical concept: even though CIN classically has low-grade proteinuria, advanced CIN with significant nephron loss can develop secondary FSGS (adaptive FSGS from hyperfiltration of remaining glomeruli), which then produces heavier proteinuria — potentially blurring the line between tubular and glomerular disease.

The Key Differentials — Compartment by Compartment

Feature	Chronic GN	CIN	Why the Difference?
Proteinuria	Heavy ( > 1 g/day, often nephrotic)	Mild ( < 1–1.5 g/day), tubular pattern	Glomerular barrier damage → albumin leakage. CIN has intact filtration barrier; only small proteins escape reabsorption
Haematuria	Dysmorphic RBCs, RBC casts	Absent or mild, non-dysmorphic	Dysmorphic RBCs squeezed through damaged GBM. CIN has no GBM damage
Urine sediment	"Active" — RBC casts	"Bland" or sterile pyuria, WBC casts	Inflammation site: glomerulus → RBCs; interstitium → WBCs
Hypertension	Early and prominent	Late (unusual early on)	Glomerular damage → salt/water retention early. CIN → salt wasting
Anaemia	Proportional to GFR	Disproportionately severe	CIN directly destroys EPO-producing interstitial cells
Complement levels	May be low (lupus nephritis, MPGN)	Normal	Complement consumption occurs at glomerular level
Kidney size	Variable (normal or small)	Small, echogenic	Both shrink in advanced disease

Key entities to distinguish from CIN:

IgA nephropathy — common in Hong Kong ^[3]; episodic painless macroscopic haematuria (brown/"smoky" urine), synpharyngitic presentation
Lupus nephritis — proteinuria/haematuria, nephrotic or nephritic syndrome; lupus nephritis occurs in ~50% of SLE patients ^[17]; but note that SLE can also cause tubulointerstitial nephritis presenting with tubular dysfunction ^[17]
Membranous nephropathy, FSGS, minimal change disease — present with nephrotic syndrome
Diabetic nephropathy — most common cause of CKD in HK (~51%) ^[5]; progressive albuminuria with diabetic retinopathy; typically nodular glomerulosclerosis (Kimmelstiel-Wilson nodules)

Exam Pitfall: SLE and the Kidney

SLE can cause BOTH glomerular disease (lupus nephritis class I–VI) AND tubulointerstitial disease. When a patient with SLE has rising creatinine with bland urinary sediment, mild proteinuria, and RTA — think lupus-associated TIN rather than worsening lupus nephritis. The management is different: TIN may respond to moderate-dose steroids whereas class IV lupus nephritis requires aggressive immunosuppression.

Feature	Vascular CKD	CIN
Hypertension	Severe, often resistant/refractory	Mild or absent early
Proteinuria	Variable, often moderate	Mild, tubular
Vascular risk factors	DM, smoking, dyslipidaemia, age	May be absent
Imaging	Renal artery stenosis, vascular calcification	Small echogenic kidneys without specific vascular findings
Flash pulmonary oedema	Characteristic of bilateral renal artery stenosis	Not typical

Key entities:

Hypertensive nephrosclerosis — second most common cause of CKD in HK ^[5]; benign (arteriolar hyalinisation) or malignant (fibrinoid necrosis)
Renal artery stenosis — secondary/resistant hypertension, flash pulmonary oedema, AKI after ACEI ^[2]
Atheroembolic disease — cholesterol crystal emboli after vascular procedures; blue toes, livedo reticularis, eosinophilia

Obstructive nephropathy is exemplified by bilateral kidney stones and benign prostatic hyperplasia ^[9].

Feature	Obstructive CKD	CIN
Imaging	Hydronephrosis, dilated pelvicalyceal system ^[11]	Small echogenic kidneys (no dilatation unless obstruction is the cause of CIN)
History	BPH, recurrent stones, pelvic malignancy	Drug exposure, autoimmune disease, metabolic derangement
Mechanism	Back-pressure → tubular damage → IF/TA	Direct tubular/interstitial injury
Reversibility	Often partially reversible if relieved early	Usually irreversible once established

Chronic obstruction → dilatation of the system will cause tubular dysfunction ^[11]. If obstruction is prolonged and untreated, it causes CIN as a secondary process — so obstruction can be both a differential diagnosis OF CIN and a CAUSE of CIN.

4. Other Important Mimics

Myeloma (CRAB), monoclonal gammopathy is listed as a differential for CKD ^[5].

Light chain cast nephropathy: light chains precipitate in distal tubules with Tamm-Horsfall protein → tubular obstruction → CIN-like picture
Distinguishing feature: urine dipstick may be negative for protein (dipstick detects albumin, not light chains), but urine protein-to-creatinine ratio (UPCR) or 24-hour urine protein is elevated — this discrepancy is the classic clue ^[14]
Also: hypercalcaemia, bone pain, anaemia (CRAB criteria)
Check: serum protein electrophoresis (SPE) if clinically suspicious ^[14]

Polycystic kidney disease ^[5] — autosomal dominant, 100% penetrance, variable expressivity; PKD1 gene (Chr 16) 85%, PKD2 gene (Chr 4) 15% ^[2]

Differential diagnosis of a large kidney on imaging: PKD, infiltration (amyloidosis), obstruction ^[5]
PKD causes CKD through progressive cyst enlargement compressing normal parenchyma → secondary tubulointerstitial damage
Distinguished from CIN by: bilateral large kidneys with multiple cysts on ultrasound (CIN → small kidneys)

The entities within TIN are: (1) Acute interstitial nephritis — drugs, infections, autoimmune disease; (2) Chronic interstitial nephritis — etiology; (3) K⁺-wasting tubular disorders; (4) Renal tubular acidosis ^[8].

AIN is not so much a "differential" as a related entity on the spectrum. The distinction matters because:

Feature	AIN	CIN
Onset	Days to weeks after exposure	Weeks to months
Kidney size	Large echogenic kidneys (enlarged by oedema) ^[18]	Small kidneys (fibrosis/atrophy)
Classic triad	Fever + eosinophilia + rash (only ~10%) ^[1]^[7]	Absent
Reversibility	Often reversible if drug stopped	Usually irreversible
Histology	Oedema + inflammatory infiltrate; preserved architecture	Fibrosis + tubular atrophy; architectural distortion
RFT decline	Decline in days ^[2]	Decline in weeks ^[2]

High Yield: AIN → CIN Transition

Persistent AIN can progress to CIN ^[1]. The clinical scenario: a patient started on PPIs months ago develops rising creatinine — initial biopsy would show AIN features, but if the PPI is not stopped, repeat biopsy months later shows fibrosis and tubular atrophy (CIN). PPIs are known to cause acute TIN → AKI and chronic TIN → CKD ^[5].

ATN involves death of tubular epithelial cells; "muddy brown casts" on urinalysis are pathognomonic ^[7].

Feature	ATN	CIN
Onset	Minutes to hours (ischaemic) or days (nephrotoxic)	Weeks to months
Urine sediment	Muddy brown granular casts, epithelial casts	Sterile pyuria, WBC casts
Reversibility	Usually reversible (tubular regeneration)	Irreversible (fibrosis)
RFT decline	Minutes ^[2]	Weeks ^[2]

Axis 2: Aetiological Differential Diagnosis Within CIN

Once you've established that the CKD is tubulointerstitial in origin, you must determine the specific cause. This is where a meticulous drug history, occupational history, family history, and autoimmune screen become essential.

Category	Specific Causes	Key Distinguishing Clue
Drug-induced	Analgesics (NSAIDs, paracetamol), calcineurin inhibitors, lithium, cisplatin, tenofovir ^[1]	Drug history; temporal relationship; papillary necrosis on imaging (analgesics)
	PPIs (chronic TIN → CKD) ^[5]	Often overlooked; very common prescription
	Many drugs implicated: antibiotics (e.g. methicillin, rifampicin), NSAIDs, immunotherapy, allopurinol ^[19]^[6]
Toxin-induced	Aristolochic acid (TCM, Balkan nephropathy), lead, cadmium, mushrooms (Cortinarius) ^[1]	TCM history (HK context); occupational exposure; multi-focal uro-epithelial malignancies with aristolochic acid ^[6]
Metabolic	Nephrocalcinosis, uric acid injury, hypokalaemia, hyperoxaluria ^[1]	Serum calcium, uric acid, potassium levels; imaging for nephrocalcinosis
Autoimmune/systemic	SLE, Sjögren's, sarcoidosis, vasculitis (GPA), IgG4-related disease ^[1]^[2]	Autoimmune markers (ANA, anti-Ro/La, ANCA, IgG4 levels); systemic features (dry eyes/mouth, skin rash, lymphadenopathy)
Infection	TB, chronic pyelonephritis ^[2]	TB contact history; chest X-ray; urine AFB; recurrent UTI history
Structural	Reflux nephropathy, renal dysplasias, chronic obstruction ^[1]	Childhood UTI history; imaging showing scarring/clubbed calyces; hydronephrosis
Inherited	Nephronophthisis, medullary cystic disease, cystinosis, oxalosis ^[1]	Family history; young age of onset; specific genetic testing
Other	DM, sickle cell disease, chronic rejection (post-transplant), radiation ^[1]^[2]	Clinical context
Idiopathic	No identifiable cause	Diagnosis of exclusion

NSAID-induced nephrotic syndrome can present with minimal change GN + acute TIN + eosinophilia (40%), higher risk in elderly ^[20]. This is a specific entity where NSAIDs cause simultaneous glomerular AND tubulointerstitial disease — another reminder that compartmental boundaries are not always clean.

NSAIDs can cause renal impairment by: (1) inhibiting prostanoid production → impaired renal blood flow regulation, (2) tubulointerstitial nephritis, (3) glomerulonephritis ^[9].

Antihypertensives can also cause TIN ^[9]. This is less well-known but important.

In the Hong Kong context, always specifically ask about:

TCM use — even though aristolochic acid is banned, other nephrotoxic herbs exist; patients may not volunteer TCM use unless directly asked
OTC NSAID use — very common, patients may consider these "harmless"
PPI use — often prescribed long-term for vague dyspepsia without clear indication

High Yield Clinical Vignette

A classic exam scenario: A diabetic patient with stable CKD (creatinine 240) sees a GP for back pain. GP adds an NSAID and a new antihypertensive. One week later, creatinine jumps to 420. The rapid deterioration may be due to: (1) NSAID-induced haemodynamic AKI (COX inhibition → afferent arteriolar vasoconstriction), (2) NSAID-induced TIN, (3) NSAID-induced GN, or (4) antihypertensive-induced TIN ^[9]. This is acute-on-chronic kidney injury — an AIN superimposed on pre-existing diabetic nephropathy, potentially progressing to CIN if the drugs are not stopped.

Lithium causes: nephrogenic DI (polyuria, polydipsia in 70%), ↑risk of CKD due to chronic interstitial nephritis (monitor eGFR Q6mo), incomplete distal RTA, nephrotic syndrome ^[21].

This is important because:

Lithium use is lifelong in many bipolar patients
The CIN is progressive and may not reverse even after lithium discontinuation
Must be distinguished from other causes of CIN in psychiatric patients (who may also take multiple medications)

Differential	Proteinuria	Haematuria	BP	Anaemia vs GFR	Kidney Size	Key Distinguishing Feature
CIN	< 1.5 g/day, tubular	Minimal	Normal/low early	Disproportionate	Small	Sterile pyuria, electrolyte disturbance, concentrating defect
Chronic GN	> 1 g/day, albuminuria	Dysmorphic RBCs, casts	Early HTN	Proportional	Variable	Active urinary sediment, complement levels, serology
Diabetic nephropathy	Progressive albuminuria	Usually absent	Early HTN	Proportional	Normal→large→small	DM history, retinopathy, nodular glomerulosclerosis
Hypertensive nephrosclerosis	Moderate	Minimal	Severe HTN	Proportional	Small	Long-standing HTN, LVH, retinopathy
PKD	Mild	Macro haematuria possible	HTN	Proportional	Large bilateral	Cysts on imaging, family history
Obstructive nephropathy	Mild	Possible	Variable	Variable	Large (hydronephrosis)	Dilated PCS on imaging, BPH/stones/tumour
Myeloma kidney	Dipstick-negative but UPCR elevated	Variable	Variable	Severe	Normal	Hypercalcaemia, bone pain, SPE/UPEP positive, CRAB
Renal artery stenosis	Variable	Minimal	Severe/resistant	Proportional	Asymmetric small	Flash pulmonary oedema, AKI after ACEI, bruit
ATN	Mild	Minimal	Variable	N/A (acute)	Normal/large	Muddy brown casts, acute onset, reversible
AIN	< 1 g/day, tubular	Microscopic	Variable	N/A (acute)	Large, echogenic	Fever/rash/eosinophilia triad (10%), recent drug, reversible

The practical approach when you suspect CIN is:

History: Drug history (especially NSAIDs, PPIs, lithium, TCM), occupational exposure (lead, cadmium), autoimmune symptoms (sicca, rash, joint pain), UTI/reflux history, family history (inherited causes), radiation history
Urinalysis: Confirm tubular proteinuria pattern, sterile pyuria, absence of heavy albuminuria or dysmorphic RBCs — this points AWAY from glomerular disease
Bloods: RFT (chronicity by comparing old values), electrolytes (RTA pattern, hypokalaemia or hyperkalaemia), calcium, uric acid, SPE
Autoimmune screen: ANA, anti-Ro/La (Sjögren's), ANCA (GPA), IgG4 levels, complement
Imaging: Ultrasound — small echogenic kidneys suggest CKD; large echogenic kidneys suggest AIN; hydronephrosis suggests obstruction; large cystic kidneys suggest PKD; asymmetric small kidney suggests renal artery stenosis or reflux nephropathy ^[5]
Biopsy: When cause remains unclear — confirms CIN histologically and may identify specific aetiology (granulomata → sarcoid/TB; IgG4+ plasma cells → IgG4-RD; paucicellular fibrosis → aristolochic acid)

Someone comes in with high creatinine with a normal-sized kidney — worry about AKI (parenchymal, vascular kidney problems, glomerulonephritis etc.) — will require a kidney biopsy ^[5]. Conversely, bilateral small kidneys with elevated creatinine point to CKD — but biopsy may still be needed if the cause is unclear.

High Yield Summary — Differential Diagnosis of CIN

Compartment-based DDx: Distinguish CIN from chronic GN (heavy proteinuria, active sediment, early HTN), vascular nephropathy (severe HTN, vascular risk factors), obstructive nephropathy (hydronephrosis), and myeloma kidney (dipstick-negative proteinuria, CRAB)
CIN "fingerprint": Tubular proteinuria < 1.5 g/day, sterile pyuria, disproportionate anaemia, pronounced electrolyte disturbance, normal phosphate, normal/low BP early, small kidneys
Key entities within TIN: AIN (drugs, infections, autoimmune), CIN (multiple aetiologies), K⁺-wasting disorders, RTA ^[8]
AIN can progress to CIN if the offending agent is not removed — especially PPIs, NSAIDs
Aetiological DDx within CIN: Drug-induced (analgesics, calcineurin inhibitors, lithium, cisplatin, PPIs), toxins (aristolochic acid, lead), metabolic (nephrocalcinosis, hypokalaemia, hyperuricaemia), autoimmune (Sjögren's, sarcoidosis, SLE, IgG4-RD), structural (reflux, obstruction), inherited (nephronophthisis, cystinosis), infection (TB), idiopathic
In HK: always ask about TCM, OTC NSAIDs, and PPI use

Active Recall - Differential Diagnosis of Chronic Interstitial Nephritis

References

^[1] Senior notes: Ryan Ho Urogenital.pdf (Section 4.2 – Chronic Interstitial Nephritis) ^[2] Senior notes: Maksim Medicine Notes.pdf (p.234 – Tubulointerstitial nephritis) ^[3] Senior notes: Block A - Glomerular and Tubulo-interstitial Diseases and Acute Kidney Injury.pdf (p.9 – Differential diagnosis of haematuria) ^[5] Senior notes: Block A - Chronic Kidney Disease and its Complications.pdf (p.6–13 – Causes, clinical features, kidney sizes, drug-induced CKD) ^[6] Senior notes: Block A - Drugs and the Kidney.pdf (p.8, p.16 – Drug-induced TIN, aristolochic acid) ^[7] Senior notes: Block A - Nephrotology Teaching Clinic RTD.pdf (p.1 – ATN, AIN clinical features) ^[8] Lecture slides: GC 057. Glomerular and Tubulo-interstitial Diseases and Acute Kidney Injury.pdf (p.59 – Entities) ^[9] Senior notes: Block A – Nephrology Data Interpretation.pdf (p.1, p.11 – Aetiologies, NSAID-induced TIN) ^[11] Senior notes: Block A - Two cases of polyuria and polydipsia.pdf (p.4–5 – Obstruction, Sjögren's, nephrogenic DI) ^[14] Senior notes: Ryan Ho Critical Care.pdf (p.25–27 – AKI workup, urine findings in TIN and myeloma) ^[16] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.995 – Glomerular vs TI disease distinction) ^[17] Senior notes: Ryan Ho Rheumatology.pdf (p.70 – SLE renal manifestations including TIN) ^[18] Senior notes: Block A - Nephrotology Teaching Clinic RTD.pdf (p.19 – TB drug-induced ATIN case, large echogenic kidneys) ^[19] Lecture slides: GC 043. Drugs and the Kidney.pdf (p.15 – Drug-induced tubulo-interstitial nephritis) ^[20] Senior notes: Block A - Drugs and the Kidney.pdf (p.14 – NSAID-induced nephrotic syndrome + AKI) ^[21] Senior notes: Ryan Ho Psychiatry.pdf (p.53 – Lithium renal side effects)

Diagnostic Criteria, Algorithm, and Investigations for Chronic Interstitial Nephritis

Unlike conditions such as SLE (ACR/EULAR criteria) or AKI (KDIGO criteria), CIN does not have a universally accepted set of diagnostic criteria with sensitivity/specificity thresholds. This is because:

CIN is a histopathological diagnosis — the definitive diagnosis rests on renal biopsy showing the characteristic triad of chronic inflammatory infiltrate (lymphocytes, plasma cells, macrophages), interstitial fibrosis, and tubular atrophy ^[7]^[22]
CIN encompasses a heterogeneous group of aetiologies, each with different clinical presentations
In practice, CIN is often diagnosed clinically (without biopsy) when the clinical picture fits — because biopsy of small, fibrotic kidneys carries higher risk and may not change management

That said, the diagnosis of CIN is built on a convergence of clinical, laboratory, and imaging findings that together form a recognisable pattern:

Working Diagnostic Framework for CIN

Domain	Criteria	Rationale
Duration	Evidence of kidney dysfunction for > 3 months (fulfilling CKD definition) ^[5]	Distinguishes from AIN (days) and ATN (minutes to hours)
Urinalysis pattern	Sterile pyuria, WBC casts, tubular proteinuria < 1–1.5 g/day, absence of significant haematuria or RBC casts ^[1]^[14]	Points to tubulointerstitial rather than glomerular origin
Biochemical pattern	Disproportionate electrolyte/acid-base disturbance for degree of GFR loss; possible RTA; phosphate may be normal ^[1]	Reflects tubular dysfunction out of proportion to glomerular damage
Haematological	Anaemia disproportionate to GFR decline (NcNc) ^[1]	EPO-producing interstitial fibroblasts directly damaged
Imaging	Bilateral small echogenic kidneys without hydronephrosis or cysts ^[5]	Fibrosis → kidney shrinkage
Histology (gold standard)	Interstitial fibrosis, tubular atrophy, chronic inflammatory infiltrate (lymphocytes + plasma cells + macrophages); glomeruli relatively spared ^[7]^[22]	Definitive but not always obtainable
Identifiable cause	Drug history, toxin exposure, metabolic derangement, autoimmune disease, structural abnormality	Aetiological diagnosis within CIN

Important Distinction: AIN vs CIN on Histology

Acute TIN: interstitial inflammation with neutrophils + lymphocytes ± eosinophils; interstitial oedema; tubulitis with focal tubular necrosis ^[22].

Chronic TIN: interstitial inflammation with lymphocytes + plasma cells + macrophages; interstitial fibrosis; tubulitis with focal tubular atrophy ^[22].

The shift from oedema → fibrosis, and from neutrophils/eosinophils → lymphocytes/plasma cells, marks the transition from acute to chronic. This is why early intervention in AIN (removing the drug) can prevent irreversible CIN.

The clinical approach to diagnosing CIN follows a logical stepwise process: first establish that CKD exists, then localise the compartment (tubulointerstitial), then identify the aetiology.

Diagnostic approach to renal diseases: History (comorbidities, drug history), Physical examination (ballottement of kidneys, bilateral pitting ankle oedema, café au lait complexion, BP measurement), Investigations — Blood tests (creatinine, urea, eGFR; metabolic/electrolyte disturbance), Urine tests (microscopy, urine protein quantification, urine culture, creatinine clearance) ^[9].

3. Investigation Modalities — Detailed Breakdown

3.1 Urinalysis

This is the single most important initial investigation for compartment localisation. It tells you whether you're dealing with a glomerular or tubulointerstitial process.

Urine dipstick detects albumin ≥ 300 mg/day (macroalbuminuria) only; cannot detect albumin < 300 mg/day (microalbuminuria) or Bence-Jones protein ^[23].

Parameter	Finding in CIN	Interpretation
Protein	Trace to 1+ (mild)	Dipstick primarily detects albumin; tubular proteinuria (low MW proteins like β2-microglobulin, α1-microglobulin) is poorly detected by dipstick → the actual protein loss may be underestimated
Blood	Negative or trace	No significant glomerular barrier disruption
Leukocyte esterase	Positive	Infection (UTI) or inflammation (e.g. AIN) ^[23] — in CIN, reflects interstitial inflammation WITHOUT infection
Nitrites	Negative	Nitrites indicate Enterobacteriaceae (UTI); negative in CIN confirms sterile process
pH	May be persistently alkaline ( > 5.5)	If distal RTA present — inability to acidify urine below pH 5.5
Glucose	May be positive (normoglycaemic glucosuria)	If proximal tubular damage → Fanconi syndrome (failed reabsorption of glucose)

Urine microscopy interpretation ^[24]:

Proteinuria + haematuria + dysmorphic RBCs + RBC casts → proliferative glomerular disease (nephritic pattern)
Heavy proteinuria + no haematuria → non-proliferative glomerular disease (nephrotic pattern) / DM nephropathy / amyloidosis
Granular or epithelial cell casts + renal tubular epithelial cells → tubulointerstitial nephritis / ATN
Pyuria → UTI / interstitial nephritis / nephrolithiasis

Finding	Significance in CIN	Pathophysiological Basis
Sterile pyuria	Characteristic finding ^[14] — WBCs in urine without bacterial growth	Inflammatory cells migrate from inflamed interstitium through damaged tubular epithelium into tubular lumen; no microbiological aetiology
WBC casts	Confirms intrarenal origin of WBCs	Tamm-Horsfall protein secreted by thick ascending limb traps WBCs in the tubular lumen → forms casts
Renal tubular epithelial (RTE) cells	Tubular damage marker	Shed from damaged tubular epithelium
Absence of dysmorphic RBCs	Excludes glomerular bleeding	Dysmorphic RBCs form by squeezing through damaged GBM — not happening in CIN
Absence of RBC casts	Excludes active glomerulonephritis	RBC casts form in Bowman's space — irrelevant in CIN

Sterile pyuria (presence of leukocytes but negative culture) differential: prostatitis, inflammation (interstitial nephritis, papillary necrosis), renal TB (repeat using early morning urine) ^[23].

Sterile Pyuria — Don't Assume UTI!

A common exam mistake: seeing WBCs in urine and reflexively thinking UTI. In CIN, the WBCs reflect inflammation without microbiological aetiology ^[3]. The urine culture is negative. If you see sterile pyuria + rising creatinine + bland sediment otherwise → think TIN, not UTI. If TB is suspected, send early morning urine (EMU) for AFB culture ^[25].

Test	Expected Finding in CIN	Interpretation
Spot urine ACR (albumin-to-creatinine ratio)	Mildly elevated	Modest albuminuria — reflects some secondary glomerular damage (adaptive FSGS) rather than primary glomerular disease
Spot urine PCR (protein-to-creatinine ratio)	Elevated, but typically PCR < 100 mg/mmol ^[7]	Tubular proteinuria — low MW proteins dominate; total protein may exceed albumin
24-hour urine protein	< 1–1.5 g/day ^[1]	Subnephrotic range. If > 3.5 g/day → reconsider glomerular disease
Urine protein electrophoresis (UPE)	Tubular pattern (β2-microglobulin, α1-microglobulin bands)	Distinguishes tubular proteinuria from glomerular (albumin-dominant) or overflow (light chain) proteinuria

Dipstick-Negative but UACR/PCR Elevated: Think Beyond CIN

If the dipstick is negative for protein but UACR is markedly elevated, consider multiple myeloma: undetectable urine protein by dipstick but ↑↑UACR ^[14]. Light chains are not detected by dipstick (which reads albumin). Always check SPE/UPEP if this discrepancy is found.

3.2 Blood Tests — Renal Function and Electrolytes

Components of RFT: sodium, chloride, potassium, urea, creatinine, creatinine clearance ^[26].

Parameter	Expected Finding	Interpretation
Serum creatinine	Elevated (chronic)	Biomarkers used in HK are serum creatinine and urine volume; HK uses μmol/L ^[26]
eGFR	Reduced	Calculated by CKD-EPI formula; stages CKD severity
Urea	Elevated	Less specific than creatinine; influenced by diet, GI bleeding, catabolism
Urea:Creatinine ratio	40–100 (normal range) or < 40 → intrinsic renal damage ^[23]	In CIN, both urea and creatinine rise proportionally (unlike pre-renal where urea rises disproportionately: ratio > 100)
Serum Na⁺	May be low (hypovolaemic hypoNa) or high-normal	Salt wasting from tubular damage → hypovolaemia → ↓Na⁺ ^[1]; or if concentrating defect → water loss → hyperNa⁺
Serum K⁺	Often elevated (hyperK⁺) ^[1] — but may be low in specific causes	HyperK⁺ if Type 4 RTA (aldosterone resistance); hypoK⁺ if proximal RTA, Sjögren's (distal RTA), or Fanconi syndrome
Serum bicarbonate	Low (metabolic acidosis)	RTA component → NAGMA ^[3]; or HAGMA if advanced CKD with organic acid retention
Serum phosphate	May be NORMAL ^[1]	Damaged proximal tubules cannot reabsorb phosphate → phosphaturia → serum PO₄ stays normal despite reduced GFR. This is a key distinguishing feature from glomerular CKD where PO₄ is elevated
Serum calcium	May be low	CKD → ↓1,25-dihydroxyvitamin D → ↓calcium absorption; plus nephrocalcinosis may be both cause and consequence

Likely AKI if normal calcium and normal phosphate (i.e., the CKD-MBD changes haven't set in yet) ^[9]. Conversely, if calcium is low and phosphate is high with elevated PTH → suggests chronicity. But in CIN, phosphate may remain normal even in chronic disease — this is a trap.

Finding	Pattern	Significance
Normal anion gap metabolic acidosis (NAGMA)	pH low, HCO₃⁻ low, AG normal (8–12)	Renal tubular acidosis — Type 1 (distal), Type 2 (proximal), or Type 4 (hyperkalaemic) ^[3]. Calculate AG to distinguish from HAGMA
High anion gap metabolic acidosis (HAGMA)	AG > 12	Seen in advanced CKD (GFR < 15) when organic acids accumulate

To distinguish RTA types in CIN:

RTA Type	Serum K⁺	Urine pH	Mechanism	Typical CIN Cause
Type 1 (distal)	Low	> 5.5 (cannot acidify)	Cannot secrete H⁺ in collecting duct	Sjögren's, lithium, nephrocalcinosis
Type 2 (proximal)	Low	< 5.5 (can acidify when HCO₃⁻ below threshold)	Cannot reabsorb HCO₃⁻ in PCT	Fanconi syndrome (cisplatin, tenofovir, lead)
Type 4	High	< 5.5	Aldosterone deficiency/resistance	Diabetic CIN, calcineurin inhibitors, obstruction

Finding	Significance
Normochromic normocytic anaemia (NcNc) — disproportionate to GFR ^[1]^[5]	EPO-producing interstitial fibroblasts destroyed → lack of erythropoietin ^[5]
Eosinophilia	If present, raises suspicion for ongoing drug-related allergic component (more typical of AIN but may persist in transition to CIN)

Test	Indication	Expected Finding in CIN
Serum calcium + PTH	Nephrocalcinosis / CKD-MBD assessment	Elevated Ca²⁺ → nephrocalcinosis as cause; low Ca²⁺ + high PTH → secondary hyperparathyroidism (consequence of CKD)
Serum uric acid	Urate nephropathy	Elevated → chronic urate crystal deposition
ANA	SLE screen	Positive in lupus TIN
Anti-Ro/SSA + Anti-La/SSB	Sjögren's syndrome ^[24]	Positive → Sjögren's-associated CIN with distal RTA
ANCA	Vasculitis (GPA)	c-ANCA/PR3 → GPA-associated granulomatous TIN
IgG4 levels	IgG4-related disease	Elevated → IgG4-related TIN
Complement (C3/C4)	SLE, MPGN	Low in lupus; normal in most CIN
SPE / serum free light chains	Myeloma ^[14]	Monoclonal band → myeloma cast nephropathy
Anti-GBM antibodies	Goodpasture's (if pulmonary-renal syndrome)	Typically negative in CIN
Lithium level	Lithium nephropathy	Therapeutic range 0.4–1.0 mmol/L; toxicity > 1.5 mmol/L
ACE level	Sarcoidosis	Elevated (non-specific)
Lead/cadmium levels	Heavy metal nephropathy	Elevated blood lead or urinary cadmium
HbS screening	Sickle cell disease	If clinically suspected (ethnic background)

Drug history is crucial — e.g. penicillins / proton pump inhibitors / NSAIDs / hydrochlorothiazide ^[9]. NSAIDs can cause renal impairment, TIN, and GN ^[9].

Test	CIN Finding	Interpretation
Fractional excretion of sodium (FENa)	> 1% ^[1]	Damaged tubules cannot reabsorb Na⁺ properly; distinguishes from pre-renal (FENa < 1%)
Urine osmolality	Low (< 300 mOsm/kg), fixed	Impaired concentrating ability — medullary damage disrupts countercurrent mechanism
Urine Na⁺	Often > 20 mmol/L	Salt wasting; unable to conserve sodium
Urine K⁺	Variable	High if K⁺-wasting (distal RTA); low if Type 4 RTA
Urine pH	> 5.5 if distal RTA	Cannot acidify urine below 5.5 → diagnostic of Type 1 RTA

3.4 Imaging

Ultrasound is the best, most non-invasive way to assess and visualise kidneys ^[5].

Finding	Interpretation	Mechanism
Bilateral small kidneys ( < 10 cm)	CKD — dysplastic, scarred, or shrunken kidney ^[5]	Fibrosis → loss of parenchyma → shrinkage
Increased echogenicity	Fibrosis / chronic disease	Fibrotic tissue reflects ultrasound more than normal parenchyma
Loss of corticomedullary differentiation	Advanced CIN	Normal cortex (hypoechoic) and medulla (relatively hyperechoic) distinction is lost as fibrosis obliterates both
Normal-sized kidneys with high creatinine	Worry about AKI — parenchymal, vascular problems; will require kidney biopsy ^[5]	Chronicity would shrink the kidneys; normal size suggests acute or subacute process
Large echogenic kidneys	AIN (enlarged by oedema) ^[18]	Interstitial oedema expands the kidneys
Hydronephrosis	Obstructive nephropathy	Back-pressure dilatation of pelvicalyceal system
Calcified papillae	Analgesic nephropathy / papillary necrosis	Necrotic papillae undergo dystrophic calcification
Nephrocalcinosis	Metabolic CIN (hypercalcaemia, distal RTA, hyperoxaluria)	Calcium deposition in medullary interstitium visible as hyperechoic medullary pyramids
Cortical scarring with calyceal clubbing	Reflux nephropathy	Chronic VUR → focal scarring, especially at poles
Asymmetric kidney size	Reflux nephropathy (scarred side smaller) or renal artery stenosis	Unilateral damage → differential size

Next investigation to do for renal impairment: renal ultrasound — does the patient only have 1 kidney? What is the size? Are the kidneys polycystic? Consider renal biopsy ^[11].

In the presence of renal impairment, try avoiding a contrast CT to prevent toxicity ^[11].

Indication	Finding
Suspected nephrocalcinosis (better sensitivity than USG)	Medullary calcification pattern
Papillary necrosis / calcified papillae	Calcified sloughed papillae; ring shadows in calyces
Renal stones as cause of obstruction	Radio-opaque calculi
Aristolochic acid nephropathy	Small kidneys ± urothelial thickening (raises concern for urothelial malignancy)

Used when urothelial malignancy is suspected (e.g., aristolochic acid exposure)
Evaluates for filling defects in renal pelvis, ureters, or bladder
In AAN patients, some underwent prophylactic removal of the urinary tract to prevent malignancy ^[6]

Nuclear medicine study — best assessment of renal scarring and differential function
Particularly useful in reflux nephropathy — identifies cortical scars that may not be visible on USG
Can quantify split renal function (% contribution of each kidney)

For definitive diagnosis of vesicoureteral reflux in suspected reflux nephropathy ^[1]
Grading of VUR (I–V) guides management

3.5 Renal Biopsy

Renal biopsy is the gold standard for definitive diagnosis of TIN ^[7]^[22].

Renal biopsy indications in the context of TIN ^[1]^[24]:

Indication	Reasoning
Diagnosis unclear — no identifiable drug/toxin/autoimmune cause	Need histological diagnosis to guide management
Considering immunosuppressive therapy	Must confirm diagnosis before starting steroids/immunosuppression
Not improving after drug withdrawal	Need to assess degree of chronicity (fibrosis vs. active inflammation) → impacts treatment decisions
Suspected specific aetiologies requiring histological confirmation	Sarcoidosis (granulomata), IgG4-RD (storiform fibrosis, IgG4+ plasma cells), TB (caseating granulomata)
Distinguishing AIN from CIN	Oedema + acute inflammation = AIN (reversible); fibrosis + atrophy = CIN (irreversible)

Renal biopsy is NOT needed if: (1) diagnosis is likely, (2) presumed precipitating drug can be easily withdrawn, and (3) patient improves after withdrawal ^[24].

With shrunken, atrophic kidneys: biopsy is difficult to locate / not safe; pathologist cannot determine original cause given fibrotic, sclerotic damage → no implications on management ^[3].

Modality	CIN Findings	Specific Aetiological Clues
Light microscopy	Interstitial fibrosis, tubular atrophy, chronic inflammatory infiltrate (lymphocytes, plasma cells, macrophages); glomeruli relatively spared ^[22]	Granulomata → sarcoidosis, TB, GPA, drug-induced (25% of drug AIN). "Striped fibrosis" → calcineurin inhibitor toxicity. Microcysts → lithium. Paucicellular fibrosis → aristolochic acid
Immunofluorescence	Usually negative or non-specific	Linear TBM deposits → anti-TBM antibody disease (rare). Granular TBM deposits → SLE
Electron microscopy	Tubular basement membrane thickening, loss of brush border microvilli	Rarely diagnostic for CIN specifically; more useful to exclude glomerular disease

Biopsy Interpretation: The Chronicity-Activity Distinction

The most important information from a CIN biopsy is the ratio of active inflammation to established fibrosis. If there is still significant active inflammation with relatively preserved architecture, there may be a window to intervene (steroids, remove cause). If the biopsy shows predominantly fibrosis and atrophy with minimal active inflammation, the damage is irreversible and management is purely supportive.

Category	Investigations	Key Findings Pointing to CIN
Urine	Dipstick, microscopy, spot ACR/PCR, 24h protein, UPE, urine culture, urine pH, urine Na⁺/K⁺/osmolality	Sterile pyuria, WBC casts, tubular proteinuria < 1.5g/day, low osmolality, FENa > 1%
Blood — basic	RFT (Na, K, Cl, urea, Cr, eGFR), ABG/VBG, CBC	Elevated Cr, NAGMA (RTA), hyperK⁺ or hypoK⁺, NcNc anaemia disproportionate to GFR, normal PO₄
Blood — aetiological	Ca, PO₄, PTH, uric acid, ANA, anti-Ro/La, ANCA, IgG4, complement, SPE, FLC, lithium level, ACE, lead/cadmium, HbA1c	Identifies specific cause
Blood — CKD complications	iPTH, vitamin D, lipid profile, iron studies	Stages CKD and identifies complications
Imaging	USG kidneys (first-line), non-contrast CT KUB, DMSA scan, MCUG, CT/MR urogram	Bilateral small echogenic kidneys, nephrocalcinosis, scarring, papillary necrosis, urothelial changes
Histology	Renal biopsy (if indicated)	IF/TA, chronic inflammatory infiltrate, glomerular sparing; specific patterns per aetiology
Special	EMU for AFB (renal TB), TB-PCR, cystoscopy (if urothelial malignancy suspected in AAN)	AFB+/granulomata → TB; urothelial lesions → AAN-related malignancy

General renal manifestations of drug-induced kidney disease: vasoconstriction, glomerular disease, tubular toxicity, tubulointerstitial nephritis, nephrolithiasis, crystalluria ^[5]^[27]. Drug-induced kidney disease classified by time: Acute ( < 7 days), Subacute (7–90 days), Chronic ( > 90 days) ^[5]^[27].

High Yield Summary — Diagnostic Approach to CIN

No formal diagnostic criteria set — CIN is diagnosed by convergence of clinical pattern + urinalysis + bloods + imaging ± histology
Gold standard = renal biopsy showing interstitial fibrosis + tubular atrophy + chronic inflammatory infiltrate (lymphocytes, plasma cells, macrophages) ^[22]
Urinalysis fingerprint: sterile pyuria, WBC casts, tubular proteinuria < 1.5 g/day, no dysmorphic RBCs
Blood fingerprint: NAGMA (RTA), disproportionate NcNc anaemia, normal PO₄, electrolyte disturbance
Imaging fingerprint: bilateral small echogenic kidneys on USG
Urea:Creatinine ratio < 40 → intrinsic renal damage ^[23]
FENa > 1% = tubular damage (cannot reabsorb Na⁺)
Biopsy NOT always needed — can diagnose clinically if clear drug cause, drug is stopped, and patient improves ^[24]
Biopsy IS needed if aetiology unclear, immunosuppression considered, or failure to improve
Do not biopsy very small fibrotic kidneys — high risk, pathologist cannot identify original cause ^[3]
Avoid contrast CT in renal impairment ^[11]

Active Recall - CIN Diagnosis and Investigations

References

^[1] Senior notes: Ryan Ho Urogenital.pdf (Section 4.1–4.2 – AIN diagnosis, CIN features, analgesic nephropathy) ^[3] Senior notes: Block A - Glomerular and Tubulo-interstitial Diseases and Acute Kidney Injury.pdf (p.28–29 – Evaluation of chronic GN, TIN entities) ^[5] Senior notes: Block A - Chronic Kidney Disease and its Complications.pdf (p.6–13 – CKD definition, kidney sizes, drug-induced CKD) ^[6] Senior notes: Block A - Drugs and the Kidney.pdf (p.8, p.16 – Drug-induced TIN, aristolochic acid) ^[7] Senior notes: Block A - Nephrotology Teaching Clinic RTD.pdf (p.1, p.9 – AIN/CIN clinical features, classification, proteinuria) ^[9] Senior notes: Block A – Nephrology Data Interpretation.pdf (p.1, p.7, p.11 – Diagnostic approach, lab interpretation, NSAIDs) ^[11] Senior notes: Block A - Two cases of polyuria and polydipsia.pdf (p.4 – Renal ultrasound, contrast avoidance) ^[14] Senior notes: Ryan Ho Critical Care.pdf (p.27 – AKI workup, urinalysis patterns, myeloma) ^[18] Senior notes: Block A - Nephrotology Teaching Clinic RTD.pdf (p.19 – TB drug-induced ATIN, large echogenic kidneys) ^[22] Senior notes: Block A - Nephrotology Teaching Clinic RTD.pdf (p.9 – AIN vs CIN histological classification) ^[23] Senior notes: Maksim Medicine Notes.pdf (p.205 – Urinalysis, sterile pyuria, urine dipstick interpretation) ^[24] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.928–1031 – Urinalysis interpretation, AIN diagnosis, biopsy indications) ^[25] Senior notes: Ryan Ho Respiratory.pdf (p.78 – Renal TB, sterile pyuria, EMU AFB) ^[26] Senior notes: Block A - Introduction to Renal Investigations (RFT, urine tests and US kidneys).pdf (p.1 – RFT components, KDIGO) ^[27] Lecture slides: GC 034. Chronic Kidney Disease and its Complications [update 2025].pdf (p.29 – Drug-induced kidney disease classification)

Management of Chronic Interstitial Nephritis

The management of CIN differs fundamentally from many other renal diseases: there is no single disease-modifying drug for CIN. Instead, management rests on three pillars:

Remove or treat the underlying cause (the only truly disease-modifying intervention)
Supportive/conservative CKD management (slow progression, manage complications)
Preparation for renal replacement therapy (when CKD progresses to stage 5)

Management of CIN is supportive in nature ^[1]. This is because once interstitial fibrosis and tubular atrophy (IF/TA) are established, they are irreversible — no drug can reverse scar tissue back to functioning nephrons. The fibrosis–rarefaction–hypoxia cycle is self-sustaining. So the entire management strategy is about stopping further damage and managing consequences.

Pillar 1: Remove or Treat the Underlying Cause

This is the most important and time-sensitive aspect of management. If the cause is identified and eliminated early — before fibrosis dominates — progression can be halted and sometimes partially reversed.

Management of drug-induced TIN: STOP the incriminated drug ^[6]^[28].

"Acute kidney injury, that may result in permanent damage. Management — STOP the incriminated drug → usually patients get better" ^[6].

Drug	Specific Actions	Key Points
NSAIDs	Discontinue immediately; substitute with paracetamol (short-course, low-dose) or non-NSAID alternatives for pain	NSAIDs can cause vasoconstriction, AKI, nephrotic syndrome, papillary necrosis, TIN ^[5]. Nephrologists seldom use NSAIDs; if you really have to, closely monitor kidney function ^[5]. Steroids may be less efficacious in NSAID-related AIN ^[1]
PPIs	Discontinue; switch to H2 receptor antagonist or on-demand PPI if GERD is the indication	PPIs can induce acute interstitial nephritis and are associated with CKD ^[29]. Try to discontinue long-term PPI therapy as much as possible ^[29]
Lithium	Discuss with psychiatry — may need to switch to alternative mood stabiliser (valproate, lamotrigine, quetiapine). Monitor eGFR Q6mo ^[21]	↑risk of CKD due to chronic interstitial nephritis ^[21]. CIN from lithium may persist even after discontinuation. Reducing lithium dose may slow progression if switching is not possible
Calcineurin inhibitors	Reduce dose, consider switch to mTOR inhibitor (sirolimus, everolimus) or belatacept in transplant patients	Calcineurin inhibitors cause vascular calcification, AKI, and CKD ^[5]
Cisplatin	Dose adjustment, adequate hydration pre- and post-infusion, consider switching to less nephrotoxic agent (e.g., carboplatin)	Causes tubular diseases with hypokalaemia and hypomagnesaemia ^[5]
Tenofovir disoproxil fumarate	Switch to tenofovir alafenamide (TAF) — much lower renal toxicity	TAF achieves higher intracellular concentrations at lower plasma levels → less tubular exposure
Aristolochic acid (TCM)	Absolute cessation of the offending herb ^[7]	Banned in Hong Kong ^[7]. Progression may continue even after cessation. Screen for urothelial malignancy

The Earlier You Stop, The Better

Discontinuation of analgesics can only stop progression if done at an early stage ^[1]. Once dense fibrosis is established, stopping the drug won't reverse the damage — it only prevents additional injury. This is why drug review is critical in EVERY patient with unexplained rising creatinine.

1.2 When to Consider Immunosuppression

Immunosuppression is usually not necessary, except for: drug-induced vasculitis, drug-induced muco-cutaneous manifestations, immune checkpoint inhibitor-associated AKI, drug-induced hepatitis ^[6]^[28].

GC slide: Management — STOP the incriminated drug. ? Immunosuppression (usually not necessary, except in drug-induced vasculitis, immune CPI-associated AKI?) ^[28].

This is a nuanced area. Here is the decision framework:

Early steroid treatment within 7 days of drug withdrawal improves outcome in drug-induced AIN. Indicated in all patients with suspected drug-induced AIN EXCEPT NSAIDs which are less likely to respond ^[24].

Scenario	Steroids Indicated?	Rationale
Drug-induced AIN with ongoing renal impairment after drug withdrawal (3–7 days)	Yes — if biopsy shows active inflammation	Active inflammatory infiltrate is steroid-responsive; may prevent transition to CIN
NSAID-induced AIN	Less effective	Mechanism is partly non-immunological (haemodynamic + direct toxicity); less efficacious in NSAID-related AIN ^[1]
Immune checkpoint inhibitor-associated AKI	Yes — responsive to steroids; those not given steroids did not improve* ^[6]	Off-target immune activation — T-cell mediated; requires immunosuppression
Established CIN with predominantly fibrosis (biopsy)	No	Fibrosis is irreversible; steroids suppress inflammation but cannot reverse scarring
CIN of unknown cause without biopsy	No — get biopsy first	Do not treat empirically without tissue diagnosis

Steroid regimens (for AIN component) ^[1]:

Prednisolone 1 mg/kg/day for 2–3 weeks, tapered over 3–4 weeks
Pulse IV methylprednisolone for 3 days if severe renal failure, followed by oral taper
Delayed treatment shows outcomes similar to supportive care alone ^[24] — hence the urgency

Steroids in CIN: The Critical Nuance

Students often ask: "Should I give steroids for CIN?" The answer is almost always no for established CIN — because by definition the histology shows fibrosis, not active inflammation. Steroids are only useful when there is a residual AIN component (i.e., the transition from AIN to CIN is still ongoing and there is active inflammation alongside fibrosis). This requires biopsy confirmation before committing to steroids.

Cause	Treatment	Rationale
Sarcoidosis	Prednisolone ± steroid-sparing agent (azathioprine, mycophenolate)	Non-caseating granulomata are steroid-responsive; treatment may halt progression if given before extensive fibrosis
SLE-associated TIN	Moderate-dose steroids ± mycophenolate or azathioprine	Less aggressive than class IV lupus nephritis regimen; TIN component may respond to moderate immunosuppression
Sjögren's syndrome	Treat underlying Sjögren's (hydroxychloroquine, steroids); immune-mediated TIN may achieve remission ^[11]	Graves' can be associated with autoimmune interstitial nephritis, especially in the context of Sjögren's ^[11]
IgG4-related disease	Prednisolone (highly responsive); rituximab for relapse	IgG4-RD is characteristically steroid-responsive
GPA (Wegener's)	Cyclophosphamide or rituximab + steroids (per vasculitis protocol)	Aggressive vasculitis requiring ANCA-vasculitis treatment protocol
TINU syndrome	Prednisolone (treats both uveitis and TIN)	Combined ocular and renal inflammation; usually self-limiting but steroids hasten recovery

Metabolic Cause	Treatment	Mechanism
Hypercalcaemia / nephrocalcinosis	Treat underlying cause (e.g., hyperparathyroidism → parathyroidectomy; sarcoidosis → steroids); IV saline, loop diuretics, bisphosphonates for acute hypercalcaemia	Reducing serum Ca²⁺ prevents further medullary calcium deposition
Chronic hypokalaemia	Identify and treat cause (e.g., diuretic adjustment, aldosterone antagonists); oral KCl supplementation	Restoring intracellular K⁺ reverses vacuolar change if given before fibrosis
Hyperuricaemia / urate nephropathy	Allopurinol/febuxostat to reduce serum urate	Reduces crystal deposition in medullary interstitium. Note: allopurinol may itself cause AIN — check HLA-B5801 in high-risk populations ^[6]
Hyperoxaluria	Primary: RNA interference therapy (lumasiran, nedosiran — newer agents targeting hepatic oxalate production); high fluid intake, pyridoxine (B6). Secondary: treat underlying cause (enteric hyperoxaluria → dietary oxalate restriction, calcium supplementation)	Reduces oxalate crystal deposition

Cause	Treatment
Reflux nephropathy	Treatment of recurrent infection ± prophylactic antibiotics; nephrectomy if recurrent pyelonephritis in a kidney with minimal residual function ^[1]
Chronic obstruction (BPH, stones)	Relieve obstruction: alpha-blockers/5-alpha reductase inhibitors for BPH; ureteral stenting; lithotripsy or surgery for stones

Cause	Treatment
Renal TB	Standard anti-TB regimen (RIPE: rifampicin, isoniazid, pyrazinamide, ethambutol — with renal dose adjustments); note that anti-TB drugs themselves can cause AIN ^[18] — monitor closely
Chronic pyelonephritis	Appropriate antibiotics; address underlying predisposition (VUR, obstruction)
BK virus (post-transplant)	Reduce immunosuppression (the virus reactivates due to over-immunosuppression)

Pillar 2: Supportive / Conservative CKD Management

Once the treatable cause is addressed (or if no reversible cause is found), management follows standard CKD care principles. CIN-related CKD has some specific considerations.

Chronic kidney disease → ACEI or ARB ^[30] for renoprotection.

Aspect	Detail	Rationale
Target	< 130/80 mmHg (KDIGO 2021)	Slows CKD progression
First-line agent	ACEI or ARB ^[30]	Reduce intraglomerular pressure by dilating efferent arteriole → slows hyperfiltration injury to remaining nephrons. Also reduce proteinuria (even tubular proteinuria)
Monitoring	Check creatinine and K⁺ within 1–2 weeks of starting	ACEI/ARB may cause initial creatinine rise (acceptable if < 30% from baseline) and hyperkalaemia (especially relevant in CIN where K⁺ is already often elevated due to Type 4 RTA)
Contraindications	Bilateral renal artery stenosis ^[9]; never prescribe ACEI and ARB together ^[9]; pregnancy; severe hyperkalaemia; eGFR < 15 (relative)	In bilateral RAS, the kidney relies on angiotensin II to maintain GFR via efferent arteriolar constriction — blocking this collapses GFR
Caution in CIN	CIN patients with salt wasting may be volume-depleted → higher risk of ACEI/ARB-induced AKI	Ensure adequate volume status before starting; start at low dose

CIN-Specific BP Consideration

Unlike glomerular CKD where hypertension is early and prominent, CIN patients may have normal or LOW BP early due to salt wasting ^[1]. These patients may not need antihypertensives initially. In fact, aggressive BP lowering in a salt-wasting CIN patient can precipitate AKI from hypoperfusion. May have superimposed AKI due to inadequate perfusion ^[1]. Always assess volume status before treating BP.

CIN causes pronounced electrolyte disturbance ^[1] — often more severe than expected for the GFR. This requires active management:

Problem	Treatment	Mechanism / Rationale
Hyperkalaemia (Type 4 RTA)	Dietary K⁺ restriction, loop diuretics (furosemide), sodium polystyrene sulfonate (Kayexalate), sodium zirconium cyclosilicate (Lokelma) or patiromer; reduce/stop ACEI/ARB if severe	Damaged collecting duct cannot secrete K⁺; aldosterone resistance
Hypokalaemia (Type 1/2 RTA, Sjögren's)	Oral KCl supplementation; treat underlying cause	Excessive urinary K⁺ loss from tubular dysfunction
Metabolic acidosis (NAGMA)	Oral sodium bicarbonate 1–2 g TDS	Replaces lost bicarbonate. Also shown to slow CKD progression (KDIGO 2024 recommends maintaining serum HCO₃⁻ ≥ 22 mmol/L). Caution: sodium load may worsen hypertension/oedema
Hyponatraemia (salt wasting)	Oral salt supplementation; adequate fluid intake	Damaged tubules cannot reabsorb Na⁺; patients may need dietary salt rather than restriction (opposite to most CKD!)
Hypomagnesaemia	Oral magnesium supplementation	Cisplatin, calcineurin inhibitors damage Mg²⁺ reabsorption in thick ascending limb

Salt Restriction vs Salt Supplementation in CIN

In most CKD (glomerular), you restrict salt because the kidneys retain it → oedema, hypertension. In CIN with salt wasting, you may need to supplement salt to prevent hypovolaemia. This is counterintuitive and frequently examined. Always assess whether the patient is volume-depleted or volume-overloaded before deciding.

CKD anaemia: normochromic normocytic anaemia → lack of erythropoietin ^[5].

CIN causes prominent anaemia that may be more severe than the degree of GFR reduction ^[1] because the EPO-producing interstitial fibroblasts are directly destroyed.

Treatment	Indication	Details
Erythropoiesis-stimulating agents (ESA)	Hb < 10 g/dL with symptoms, or Hb < 9 g/dL regardless	Recombinant EPO (epoetin alfa, darbepoetin alfa); target Hb 10–11.5 g/dL — do NOT aim for normal Hb (↑cardiovascular risk). Administer subcutaneously
Iron supplementation	Ferritin < 100 ng/mL or TSAT < 20%	Oral iron (ferrous sulphate) if tolerated; IV iron (iron sucrose, ferric carboxymaltose) if oral not tolerated or if on ESA. Must replete iron stores before ESA will be effective
Investigate other causes	Before attributing anaemia solely to CKD	Rule out iron deficiency (GI blood loss — especially if on NSAIDs), B12/folate deficiency, chronic disease component

CKD-MBD: high or low PTH, bone biochemistry abnormality, vascular calcification ^[5].

An interesting point about CIN: phosphate may be normal due to impaired tubular reabsorption ^[1]. This means secondary hyperparathyroidism may develop via a different pathway — predominantly through ↓1,25-dihydroxyvitamin D (the kidneys cannot hydroxylate 25-OH vitamin D to active form) rather than through phosphate retention.

Treatment	Indication	Details
Phosphate binders	Serum PO₄ > 1.45 mmol/L (may be less needed in CIN)	Calcium carbonate (with meals), sevelamer, lanthanum carbonate. In CIN, phosphate may stay normal longer → may not need binders until late
Active vitamin D (calcitriol / alfacalcidol)	Elevated PTH with ↓calcium	Replaces the active 1,25(OH)₂D₃ that the damaged kidney cannot produce. Monitor Ca²⁺ to avoid hypercalcaemia
Calcimimetics (cinacalcet)	Severe secondary/tertiary hyperparathyroidism not responding to vitamin D	Activates calcium-sensing receptor on parathyroid gland → ↓PTH secretion

Must monitor kidney function for many drugs → metformin, ACEI/ARB ^[5].

Nephrotoxin to Avoid	Why
NSAIDs	Nephrologists seldom use this ^[5]; further haemodynamic injury + direct tubulotoxicity
Aminoglycosides	Cause AKI ^[5]; concentrated in tubular cells; use alternatives or dose-adjust with monitoring
Iodinated contrast	In the presence of renal impairment, try avoiding contrast CT to prevent toxicity ^[11]; use non-contrast imaging where possible. If contrast essential: pre-hydration with IV normal saline, use iso-osmolar contrast, minimise volume
Metformin	Risk of lactic acidosis in advanced CKD (eGFR < 30 → contraindicated; 30–45 → reduce dose)
Further TCM without verification	Ongoing risk of aristolochic acid or other nephrotoxic herbs
Nephrotoxic combination therapy	Never prescribe ACEI and ARB together ^[9]

Recommendation	Rationale
Moderate protein restriction (0.6–0.8 g/kg/day) in CKD stages 3b–5	Reduces urea generation, slows hyperfiltration of remaining nephrons
Potassium restriction if hyperkalaemic	Reduce K⁺ load when tubular K⁺ secretion is impaired
Salt: INDIVIDUALISED	Unlike most CKD (restrict salt) — CIN with salt wasting may need normal or increased salt intake. Assess volume status
Adequate fluid intake	CIN patients with concentrating defect need to drink enough to match their obligatory water losses (polyuria)
Phosphate restriction if hyperphosphataemic	Standard CKD measure; may be less critical in CIN where phosphate is often normal

CKD from any cause carries markedly elevated cardiovascular risk. Standard measures include:

Statin therapy (atorvastatin preferred — does not require renal dose adjustment)
Glycaemic control if diabetic (HbA1c target 7%; avoid metformin if eGFR < 30)
Smoking cessation
Weight management
SGLT2 inhibitors (dapagliflozin, empagliflozin) — DAPA-CKD and EMPA-KIDNEY trials showed benefit in CKD regardless of diabetes. These agents reduce intraglomerular pressure (afferent arteriolar constriction via tubuloglomerular feedback) and have anti-fibrotic/anti-inflammatory properties. Now recommended by KDIGO 2024 for CKD with eGFR > 20 and ACR > 20 mg/mmol.

SGLT2 Inhibitors in CIN — Practical Considerations

SGLT2 inhibitors have shown CKD-protective benefits across multiple aetiologies. However, their evidence base is strongest for diabetic and proteinuric CKD. In CIN with minimal proteinuria, the evidence is less robust. They should still be considered for patients meeting KDIGO criteria, but caution is needed in volume-depleted, salt-wasting CIN patients (risk of hypotension and hypovolaemia). Start low and monitor.

Parameter	Frequency	Rationale
eGFR, creatinine	Every 3–6 months (more frequently if declining)	Track CKD progression
Electrolytes (Na, K, HCO₃⁻, Ca, PO₄, Mg)	Every 3–6 months	Detect and manage tubular dysfunction complications
CBC, reticulocyte count, iron studies	Every 3–6 months	Monitor anaemia and ESA response
PTH, vitamin D	Every 6–12 months (stage 3–4); every 3 months (stage 5)	CKD-MBD monitoring
Urine ACR / PCR	Every 6–12 months	Track proteinuria (even mild tubular proteinuria guides ACEI/ARB use)
Blood pressure	Every visit	Hypertension control
Lithium patients: eGFR Q6mo ^[21]	Every 6 months	Lithium-induced CIN monitoring
Screening for urothelial malignancy (aristolochic acid exposure)	Annually — urine cytology, cystoscopy, CT/MR urogram	Increased risk of urothelial carcinomas ^[7]

Preparation of renal replacement therapy if approaching stage 5 CKD ^[3].

Aspect	Detail
When to prepare	eGFR < 20 → refer for RRT planning; eGFR < 15 or symptoms of uraemia → consider initiating RRT
RRT options	Peritoneal dialysis (PD), haemodialysis (HD), renal transplantation
Vascular access	Arteriovenous fistula (AVF) creation ideally 6 months before anticipated HD start
Transplant evaluation	CIN patients are generally good transplant candidates (the native kidney disease won't recur unless the cause is systemic/ongoing). Special consideration: aristolochic acid nephropathy → prophylactic bilateral nephroureterectomy before transplant due to urothelial malignancy risk

Indications for dialysis (mnemonic: AEIOU) ^[14]:

Acidosis: pH < 7.1 refractory to bicarbonate
Electrolyte: hyperK > 6.5 refractory to medical Rx
Intoxication: drug removal
Overload: fluid overload refractory to diuretics
Uraemia: pericarditis, neuropathy, encephalopathy

Special Management Scenarios

CIN patients may have superimposed AKI due to inadequate perfusion ^[1]. This occurs because salt-wasting patients become volume-depleted, or when nephrotoxic drugs (especially NSAIDs, ACEI/ARB) are added to a patient with pre-existing CIN.

Management:

Identify and remove the precipitant (drug, dehydration, infection)
Volume resuscitation (cautious — these patients have reduced nephron mass)
Review medications: ALWAYS done in AKI ^[14]
Monitor for recovery — the acute component may reverse, returning to the baseline CKD level

Characterised by papillary necrosis and CIN ^[1].

Papillary sloughing can cause: flank pain, ureteric colic, haematuria, ureteral obstruction
Acute management: analgesia (NOT NSAIDs!), IV fluids, ureteral stenting if obstructed
Long-term: discontinuation of analgesics can only stop progression if done at an early stage ^[1]
Screen for urothelial CA (↑risk with chronic analgesic use)

↑risk of CKD due to chronic interstitial nephritis; monitor eGFR Q6mo ^[21].

Shared decision-making with psychiatry is essential
If eGFR is declining: consider switching to alternative mood stabiliser
If lithium must continue (e.g., refractory bipolar disorder): use lowest effective dose, ensure adequate hydration, avoid dehydration/sodium depletion, amiloride may reduce lithium entry into collecting duct cells (via ENaC blockade — lithium enters through ENaC)
CIN from lithium may not reverse even after stopping → manage as CKD

Management Category	Specific Interventions	Contraindications / Cautions
Remove cause	Stop offending drug; chelation for lead; treat hypercalcaemia; relieve obstruction; anti-TB; immunosuppression for autoimmune	—
Corticosteroids	For residual AIN component on biopsy; immune checkpoint inhibitor AIN; autoimmune causes	Not for established fibrosis; less efficacious in NSAID-related AIN ^[1]; DM (hyperglycaemia); infection (screen for TB first)
ACEI/ARB	BP control + renoprotection	Bilateral RAS ^[9]; severe hyperK; pregnancy; severe volume depletion in salt-wasting CIN
Sodium bicarbonate	NAGMA (target HCO₃⁻ ≥ 22)	Fluid overload; severe hypertension (sodium load)
ESA + Iron	Anaemia (Hb < 10 with symptoms)	Uncontrolled HTN (ESA can ↑BP); iron overload; active infection (defer IV iron)
SGLT2 inhibitors	CKD with eGFR > 20	Type 1 DM (DKA risk); severe volume depletion; eGFR < 20 (do not initiate, may continue if already on)
Salt supplementation	Salt-wasting CIN with hypotension	Fluid overload; hypertension
Avoid nephrotoxins	NSAIDs, aminoglycosides, contrast, TCM	—
RRT	Stage 5 CKD / AEIOU indications	Patient choice (conservative care pathway is valid for elderly/frail)

High Yield Summary — Management of CIN

Management is supportive in nature ^[1] — no disease-specific drug for CIN
Most important step: STOP the incriminated drug ^[6]^[28] — this is the mainstay of drug-induced TIN management
Immunosuppression usually NOT necessary, except for: drug-induced vasculitis, immune CPI-associated AKI ^[28], and autoimmune causes (sarcoidosis, SLE, IgG4-RD, Sjögren's)
Steroids only useful if residual active AIN component on biopsy — NOT for established fibrosis
Early steroid treatment within 7 days of drug withdrawal improves outcome; EXCEPT NSAIDs (less responsive) ^[24]
ACEI/ARB for CKD renoprotection ^[30] but cautious in salt-wasting/volume-depleted CIN patients
Electrolyte management is critical — CIN causes pronounced disturbance: oral NaHCO₃ for NAGMA, K⁺ management (direction depends on RTA type), Mg²⁺ replacement
Anaemia management: ESA + iron — anaemia is disproportionate to GFR
Discontinuation of analgesics can only stop progression if done early ^[1]
Lithium: monitor eGFR Q6mo ^[21]; amiloride may reduce lithium entry into collecting duct
Aristolochic acid: stop exposure + screen for urothelial malignancy + consider prophylactic nephroureterectomy ^[7]
Avoid nephrotoxins (NSAIDs, contrast, aminoglycosides); never combine ACEI and ARB ^[9]
Prepare for RRT if approaching stage 5 CKD — dialysis (AEIOU indications) or transplantation

Active Recall - Management of Chronic Interstitial Nephritis

References

^[1] Senior notes: Ryan Ho Urogenital.pdf (Section 4.1–4.2 – AIN treatment, CIN management, analgesic nephropathy) ^[3] Senior notes: Block A - Glomerular and Tubulo-interstitial Diseases and Acute Kidney Injury.pdf (p.28 – Chronic GN treatment principles, preparation for RRT) ^[5] Senior notes: Block A - Chronic Kidney Disease and its Complications.pdf (p.11–12 – Drug-induced CKD, CKD clinical manifestations, NSAID avoidance) ^[6] Senior notes: Block A - Drugs and the Kidney.pdf (p.8–9 – Drug-induced TIN management, immune checkpoint inhibitors, direct toxicity drugs) ^[7] Senior notes: Block A - Nephrotology Teaching Clinic RTD.pdf (p.11 – Aristolochic acid nephropathy, drug-induced TIN classification) ^[9] Senior notes: Block A – Nephrology Data Interpretation.pdf (p.11 – NSAID/antihypertensive-induced TIN, ACEI/ARB contraindications) ^[11] Senior notes: Block A - Two cases of polyuria and polydipsia.pdf (p.4 – Sjögren's-associated TIN, contrast avoidance) ^[14] Senior notes: Ryan Ho Critical Care.pdf (p.26 – AKI management, dialysis indications AEIOU, drug review) ^[18] Senior notes: Block A - Nephrotology Teaching Clinic RTD.pdf (p.19 – TB drug-induced ATIN case) ^[21] Senior notes: Ryan Ho Psychiatry.pdf (p.53 – Lithium renal side effects, monitoring, CIN risk) ^[24] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.1030–1032 – AIN treatment, steroid indications, NSAIDs less responsive) ^[28] Lecture slides: GC 043. Drugs and the Kidney.pdf (p.16 – Drug-induced TIN management, stop drug, immunosuppression usually not necessary) ^[29] Senior notes: Block A - Indigestion and 'heartburn'_ nausea and vomiting; gastric motility problems; benign esophageal lesions.pdf (p.11 – Long-term PPI risks, AIN) ^[30] Senior notes: Block A - High blood pressure_ hypertension.pdf (p.42 – ACEI/ARB for CKD)

Complications of Chronic Interstitial Nephritis

CIN is, by definition, a progressive process that leads to CKD and ultimately ESRD. The complications of CIN therefore encompass two overlapping categories:

Complications specific to the tubulointerstitial pathology (arising directly from tubular and interstitial dysfunction — these are often earlier and more prominent than in glomerular CKD)
General CKD complications (shared with all causes of CKD once GFR declines sufficiently)
Aetiology-specific complications (unique to the underlying cause of CIN)

The key teaching point: in CIN, the tubular dysfunction complications appear early and are disproportionate to the GFR loss, whereas the uraemic and filtration-failure complications appear late. This is the opposite pattern to glomerular CKD, where proteinuria and filtration failure dominate early while tubular function is relatively preserved.

1. Complications Arising from Tubular Dysfunction (Early and Prominent)

These occur because the primary site of damage — the tubules and interstitium — is responsible for fine-tuning the filtrate. When this "processing plant" fails, the electrolyte, acid-base, and water homeostasis is disrupted long before filtration itself is significantly impaired.

RTA is one of the four clinical entities encapsulated within TIN ^[3]^[8].

RTA Type	Mechanism in CIN	Clinical Consequence	Typical CIN Cause
Type 1 (distal)	Damaged α-intercalated cells in collecting duct cannot secrete H⁺ against a gradient → urine pH stays > 5.5	NAGMA + hypokalaemia (H⁺ not secreted → Na⁺ reabsorption coupled with K⁺ secretion instead) + nephrocalcinosis (alkaline urine favours calcium phosphate precipitation)	Sjögren's syndrome, lithium, amphotericin B
Type 2 (proximal)	Damaged proximal tubular cells cannot reabsorb filtered HCO₃⁻ → bicarbonate wasting	NAGMA + Fanconi syndrome features (glucosuria, aminoaciduria, phosphaturia, uricosuria)	Cisplatin, tenofovir, lead, cystinosis
Type 4 (hyperkalaemic)	Damaged cortical collecting duct cells resistant to aldosterone OR reduced aldosterone production	NAGMA + hyperkalaemia — the combination of metabolic acidosis with HIGH potassium distinguishes Type 4 from Types 1/2	Diabetic CIN, calcineurin inhibitor toxicity, obstructive nephropathy

Renal tubular acidosis → loss of both potassium and hydrogen ions ^[11]. This is specifically for Type 1 distal RTA, where H⁺ cannot be secreted, and compensatory K⁺ secretion leads to hypokalaemia.

Why RTA matters clinically: Untreated metabolic acidosis accelerates CKD progression (acid load promotes protein catabolism, bone demineralisation, and tubulointerstitial inflammation), worsens muscle wasting, and contributes to growth failure in children. Chronic hypokalaemia from Type 1 RTA can itself worsen tubular damage (hypokalaemic nephropathy — a vicious cycle).

Concentration defect (early): polyuria/nocturia ± salt wasting → due to tubular damage to medullary area ^[1].

Mechanism	Consequence
Disrupted medullary concentration gradient (countercurrent multiplication damaged by fibrosis)	Kidney cannot produce concentrated urine regardless of ADH levels
Decreased expression of AQP2 (particularly in lithium toxicity) ^[11]	Collecting duct impermeable to water even with maximal ADH stimulation
Prolonged DI → urine buildup → dilates the entire urinary pathway ^[11]	Secondary hydronephrosis, which compounds the obstruction and further damages the kidney

Clinical impact: Polyuria of 3–10 L/day → obligatory high fluid intake → dangerous if the patient cannot drink (unconscious, post-operative, intercurrent illness) → severe hypernatraemia → seizures, altered consciousness, central pontine myelinolysis (if corrected too rapidly)

May have superimposed AKI due to inadequate perfusion ^[1] — this is the acute-on-chronic complication: the salt-wasting, volume-depleted CIN patient becomes dehydrated → pre-renal AKI on top of CKD.

The Dehydration Trap in CIN

Unlike most CKD patients who retain salt and water, CIN patients with concentrating defects and salt wasting are vulnerable to volume depletion. An intercurrent illness (vomiting, diarrhoea, reduced oral intake) that would be tolerable for a patient with normal tubular function can precipitate severe AKI in a CIN patient because they cannot conserve water or sodium. Always counsel these patients about maintaining fluid intake during illness.

Electrolyte disturbance: pronounced, often hyperK and acidosis ^[1].

Electrolyte Problem	Mechanism	Clinical Consequence
Hyperkalaemia	Type 4 RTA; reduced distal K⁺ secretion; aldosterone resistance	Cardiac arrhythmias (peaked T waves → widened QRS → sine wave → VF/asystole); muscle weakness
Hypokalaemia	Type 1/2 RTA; renal K⁺ wasting (Sjögren's, Fanconi)	Cardiac arrhythmias (U waves, prolonged QT); muscle weakness, paralytic ileus, rhabdomyolysis; chronic hypokalaemia itself perpetuates tubular dysfunction ^[12]
Hyponatraemia	Salt wasting → hypovolaemic hyponatraemia	Confusion, seizures, cerebral oedema (if severe/acute)
Hypernatraemia	Concentrating defect → excessive water loss without adequate intake	Altered consciousness, seizures
Hypomagnesaemia	Impaired Mg²⁺ reabsorption in thick ascending limb (cisplatin, calcineurin inhibitors)	Refractory hypokalaemia (Mg²⁺ is required for ROMK channel function in distal nephron), arrhythmias, tetany
Phosphate may be normal ^[1]	Impaired proximal tubular PO₄ reabsorption → phosphaturia	Osteomalacia (in proximal RTA/Fanconi); however, absence of hyperphosphataemia delays the onset of secondary hyperparathyroidism (compared to glomerular CKD)

This is a complication that is also a perpetuating cause — creating a vicious cycle:

CIN (especially Type 1 RTA) → alkaline urine + hypercalciuria + hypocitraturia → calcium phosphate crystal deposition in medullary interstitium → nephrocalcinosis → further tubular damage → more CIN

Nephrocalcinosis is particularly seen in:

Distal RTA from Sjögren's syndrome
Sarcoidosis (hypercalcaemia + CIN)
Primary hyperparathyroidism

2. General CKD Complications (Shared with All Causes of CKD)

These complications develop as GFR progressively declines. In CIN, they tend to appear later than in glomerular CKD (because tubular dysfunction dominates early) — but some complications (especially anaemia) appear disproportionately early.

Normochromic normocytic anaemia → lack of erythropoietin ^[5].

Prominent anaemia: may be more severe than the degree of GFR reduction ^[1].

Why is anaemia disproportionate in CIN?

EPO is produced by peritubular interstitial fibroblasts in the renal cortex
In CIN, the interstitium is the primary target of damage → fibroblasts are replaced by fibrotic tissue → EPO production drops early and severely
In glomerular CKD, the interstitium is relatively spared until late → EPO production correlates with GFR decline
This is clinically important because CIN patients may need ESA therapy at higher GFR levels than standard CKD guidelines suggest

Consequences of anaemia: fatigue, reduced exercise tolerance, café au lait complexion (anaemia pallor + retained uraemic pigments) ^[4], exacerbation of heart failure (high-output cardiac state → LVH → HFrEF), reduced quality of life.

Systemic clinical manifestations of CKD: hypertension, LVH; congestive cardiac failure → systolic HF, HFrEF ^[5].

Complication	Mechanism	CIN-Specific Nuance
Hypertension	Late in CIN (unlike glomerular CKD); develops when cortical damage causes sodium retention and volume expansion	Hypertension is unusual until late in CIN ^[1] — but when it develops, it accelerates CKD progression. CKD is insidious; good practice for a patient presenting with HT to get a urine dipstick ^[5]
LVH	Chronic hypertension + anaemia + volume overload → concentric/eccentric LVH	The severe anaemia in CIN contributes to high-output state → eccentric LVH earlier than in non-CIN CKD
Heart failure	LVH → diastolic → systolic dysfunction; fluid overload; uraemic cardiomyopathy
Accelerated atherosclerosis	CKD is an independent cardiovascular risk factor; uraemic toxins → endothelial dysfunction; CKD-MBD → vascular calcification	Cardiovascular disease is the leading cause of death in CKD patients (not uraemia or infection)
Uraemic pericarditis	Uraemic toxins irritate the pericardium (BUN typically > 60 mg/dL)	Late complication; an indication for dialysis

CKD-MBD: high or low PTH, bone biochemistry abnormality, vascular calcification ^[5].

The pathophysiology of CKD-MBD in CIN has a unique feature:

Step	Standard CKD-MBD	CIN-Specific Difference
1. ↓GFR → ↓phosphate excretion → ↑PO₄	Hyperphosphataemia is the initial trigger	Phosphate may be NORMAL in CIN because damaged proximal tubules cannot reabsorb PO₄ → ongoing phosphaturia keeps serum PO₄ normal ^[1]
2. ↑PO₄ → ↓1,25(OH)₂D₃ → ↓Ca²⁺	↓vitamin D activation occurs in both	This step still occurs in CIN (reduced 1-alpha hydroxylase activity in damaged proximal tubules)
3. ↓Ca²⁺ → ↑PTH (secondary hyperPTH)	Yes	May be delayed in CIN (because PO₄ is not elevated, one of the PTH triggers is absent)
4. Chronic ↑PTH → bone disease	Osteitis fibrosa cystica (high-turnover); or adynamic bone disease (low-turnover) if PTH is over-suppressed
5. Vascular calcification	Calcium-phosphate product deposition in vessel walls	CKD resulting in vascular calcification is a characteristic pathological change of CKD-MBD ^[5]

Clinical consequences: bone pain, fractures, vascular calcification (coronary, peripheral → accelerated CVD), calciphylaxis (rare but devastating — calcium deposition in small dermal vessels → skin necrosis).

Excretion defect (late): uraemia, hypertension → occurs when tubular damage and fibrosis progress to the cortex ^[1].

Chronic TIN is the major pathway leading to chronic kidney disease ^[3].

Uraemic Complication	Mechanism	Key Features
Uraemic encephalopathy	Accumulation of nitrogenous waste products → CNS toxicity	Confusion, asterixis, myoclonus, seizures, coma
Uraemic pericarditis	Direct toxic effect on pericardium	Chest pain (pleuritic), pericardial friction rub; an ABSOLUTE indication for dialysis
Uraemic neuropathy	Nerve damage from uraemic toxins	Peripheral neuropathy (glove-and-stocking), restless legs syndrome
Uraemic pruritus	Multifactorial: uraemic toxins, secondary hyperPTH, skin dryness, altered nociception	Generalised itch; scratch marks on examination
Immunodeficiency	Uraemia impairs neutrophil and lymphocyte function	↑risk of infections; poor vaccine response
Bleeding tendency	Platelet dysfunction (uraemic toxins impair platelet aggregation)	Epistaxis, GI bleeding, prolonged bleeding time
Nausea/anorexia/malnutrition	Uraemic toxins → GI mucosal irritation → reduced appetite; protein catabolism from acidosis	Weight loss, muscle wasting, hypoalbuminaemia

Progression to ESRD: preparation of renal replacement therapy if approaching stage 5 CKD ^[3]. In CIN specifically, aristolochic acid nephropathy may progress to end-stage renal failure within months ^[7].

Metabolic acidosis is listed as a systemic CKD manifestation ^[5].

Beyond the RTA-type acidosis from tubular dysfunction (an early feature), advanced CKD adds a high anion gap component as organic acids (sulphate, phosphate, hippurate) accumulate due to reduced filtration. This dual-component acidosis (NAGMA early from RTA + HAGMA late from filtration failure) is characteristic of advanced CIN-related CKD.

Consequences of chronic metabolic acidosis:

Accelerated muscle protein catabolism → sarcopenia
Bone demineralisation (bone buffers acid by releasing Ca²⁺ and PO₄)
Accelerated CKD progression (acid promotes tubulointerstitial inflammation)
Growth retardation in children

3. Aetiology-Specific Complications

These are unique to the underlying cause of CIN and represent important exam-worthy associations.

Aristolochic acid is known to cause: (1) Chronic interstitial nephritis and CKD, (2) Multi-focal uro-epithelial malignancies ^[6].

Some patients had to undergo prophylactic removal of the urinary tract, to prevent malignancy coming up later ^[6].

Increased risk of urothelial carcinomas ^[7].

Mechanism: Aristolochic acid metabolites form DNA adducts → characteristic A:T → T:A transversions in the TP53 tumour suppressor gene → urothelial carcinoma of the renal pelvis, ureter, and/or bladder. The malignancy risk persists for years even after aristolochic acid cessation.

Clinical implication: Lifelong surveillance with urine cytology, cystoscopy, and CT/MR urogram in all patients with known aristolochic acid exposure. Some centres advocate prophylactic bilateral nephroureterectomy before renal transplantation.

High Yield: The Dual Pathology of Aristolochic Acid

When you see aristolochic acid / TCM nephropathy in an exam, you MUST mention both: CIN → CKD AND urothelial malignancy. This is a classic two-part answer that examiners look for. The malignancy risk is the reason prophylactic surgery is considered — you don't just manage the CKD, you screen for cancer.

Analgesic nephropathy is characterised by papillary necrosis and CIN ^[1].

Complication	Mechanism	Clinical Feature
Papillary necrosis	NSAID-induced medullary ischaemia (COX inhibition → ↓PG → vasoconstriction of vasa recta)	Flank pain, ureteric colic (sloughed papilla obstructs ureter), haematuria; ring shadow or "egg-in-cup" appearance on IVU
Urothelial carcinoma	Chronic exposure to analgesic metabolites (phenacetin metabolites) → urothelial DNA damage	↑risk of transitional cell carcinoma of renal pelvis; ↑risk of urothelial CA and atherosclerotic risk ^[1]

Lithium: nephrogenic DI → polyuria, polydipsia (70%); ↑risk of CKD due to chronic interstitial nephritis; others: incomplete distal RTA, nephrotic syndrome ^[21].

Nephrogenic DI may persist even after lithium discontinuation (irreversible downregulation of AQP2 + structural damage to medulla)
Lithium also causes hyperparathyroidism and hypercalcaemia ^[21] → which in turn can cause nephrocalcinosis → another pathway to worsen CIN

Chronic scarring → activation of RAAS in damaged segments → secondary hypertension
Anatomical distortion of pelvicalyceal system → predisposition to recurrent pyelonephritis → further scarring (vicious cycle)
End-stage: non-functioning kidney acting as nidus for infection → may require nephrectomy

Chronic lead exposure → inhibition of uric acid secretion by proximal tubule → hyperuricaemia → gout
The combination of CKD + gout + hypertension in a patient with occupational lead exposure is called "saturnine gout" (from Saturn, the alchemical symbol for lead)

Renal TB pathogenesis: MTB deposited at glomerulus → cortical granuloma → rupture into PCT → trapped in loop of Henle → medullary granuloma or chronic TIN → papillitis with gradual destruction of renal parenchyma ± cavern formation → dystrophic calcification with non-functioning kidney (auto-nephrectomy) as end-stage ^[25].

Autonephrectomy: complete calcification and non-function of the kidney — end-stage of renal TB
Ureteral strictures: downstream spread of TB → segmental strictures and dilatation ("corkscrew ureter"), shortened rigid ureter ("pipe-stem ureter") ^[25] → hydronephrosis → further renal damage
Contralateral compensatory hypertrophy: may maintain overall renal function for a time

May have superimposed AKI due to inadequate perfusion ^[1].

This is one of the most important practical complications. CIN patients are vulnerable to AKI from multiple mechanisms:

Precipitant	Mechanism	Why CIN Patients Are Particularly Vulnerable
Volume depletion (vomiting, diarrhoea, poor oral intake)	Pre-renal AKI	Salt wasting + concentrating defect = obligatory fluid losses; reduced renal reserve to compensate
NSAIDs	Haemodynamic AKI + direct tubulotoxicity	NSAIDs can cause AKI, nephrotic syndrome, papillary necrosis, TIN ^[5]; already-damaged kidneys are extremely vulnerable
ACEI/ARB in volume-depleted state	↓efferent arteriolar tone → ↓intraglomerular pressure → ↓GFR	ACEI and ARB contraindicated in bilateral RAS; never combine ^[9]; CIN patients with reduced nephron mass are functionally similar
Contrast media	Contrast-induced AKI (medullary ischaemia + direct tubulotoxicity)	In the presence of renal impairment, try avoiding contrast CT ^[11]
Infection (UTI/sepsis)	Inflammatory → haemodynamic → AKI	Structural abnormalities in reflux nephropathy predispose to UTI
New drugs added	AIN superimposed on CIN	Any new drug (PPI, antibiotic, allopurinol) can trigger AIN on top of existing CIN

Drug-induced TIN may lead to permanent damage ^[28]. An episode of AKI on a CIN background may cause irreversible loss of the remaining functional nephrons, accelerating the trajectory toward ESRD.

The ultimate complication of CIN is progression to end-stage renal failure ^[7]. In some aetiologies this is rapid (aristolochic acid: months) while in others it is slow (reflux nephropathy: decades). Once ESRD is reached, complications relate to:

Need for renal replacement therapy: dialysis (PD or HD) or transplantation, each with their own complication profiles
Chronic allograft injury post-transplant: a clinicopathological syndrome characterised by progressive decline in renal graft function, proteinuria, hypertension, with histological features of interstitial fibrosis and tubular atrophy ^[10] — the leading cause of late graft failure after renal transplant ^[10]. Essentially, the transplanted kidney can develop a form of CIN (from calcineurin inhibitor toxicity, chronic rejection, or other causes)
Quality of life: dialysis dependence, dietary restrictions, vascular access complications, peritonitis (PD)

Category	Complication	Timing	CIN-Specific Feature
Tubular dysfunction	RTA (Types 1, 2, 4)	Early	Disproportionate to GFR; NAGMA with variable K⁺
	Nephrogenic DI	Early	Polyuria, nocturia; risk of hypernatraemia; may persist after cause removed (lithium)
	Electrolyte disturbance	Early	Pronounced; hyperK or hypoK; hypoMg; hypoNa
	Salt wasting → volume depletion	Early	Risk of superimposed pre-renal AKI
	Nephrocalcinosis (secondary)	Early-Mid	Vicious cycle: RTA → alkaline urine → Ca deposition → more TI damage
Haematological	Disproportionate anaemia	Early-Mid	EPO-producing fibroblasts directly damaged
Cardiovascular	Hypertension, LVH, HF	Late	HTN unusual until late in CIN (vs early in glomerular CKD)
	Accelerated atherosclerosis	Late	CKD is independent CV risk factor
	Uraemic pericarditis	Very late	Indication for dialysis
CKD-MBD	Secondary hyperPTH, bone disease	Mid-Late	PO₄ may be normal (delayed hyperPTH trigger); vascular calcification
Uraemia	Encephalopathy, neuropathy, pruritus, bleeding	Very late	When GFR < 15
Aetiology-specific	Urothelial malignancy (aristolochic acid, analgesics)	Any time	Must screen lifelong; prophylactic surgery may be needed
	Papillary necrosis (analgesics)	Mid	Flank pain, colic, haematuria, ureteral obstruction
	Autonephrectomy (TB)	Late	Complete calcification and non-function
	Saturnine gout (lead)	Mid	Gout + CKD + HTN in lead-exposed patient
Acute-on-chronic	Superimposed AKI	Any time	Salt wasting + concentrating defect → volume-depletion vulnerability
End-stage	ESRD requiring RRT	Late	Irreversible; transplant graft may itself develop IF/TA (chronic allograft injury)

High Yield Summary — Complications of CIN

Tubular dysfunction complications appear EARLY and are DISPROPORTIONATE to GFR — this distinguishes CIN from glomerular CKD
RTA (Types 1, 2, or 4) is one of the four TIN entities ^[3]^[8] — NAGMA with variable K⁺ depending on type
Concentrating defect → polyuria/nocturia → risk of hypernatraemia and volume depletion → superimposed AKI ^[1]
Anaemia: disproportionately severe for GFR ^[1] because EPO-producing interstitial fibroblasts are directly damaged — may need ESA at higher GFR than standard CKD
Phosphate may be NORMAL ^[1] → delays secondary hyperPTH; a distinctive feature of CIN
Aristolochic acid: dual complication of CIN + urothelial malignancy ^[6] — requires lifelong cancer surveillance and possibly prophylactic nephroureterectomy
Analgesic nephropathy: papillary necrosis + CIN + ↑risk of urothelial CA ^[1]
Lithium: nephrogenic DI (70%) + CIN + hyperparathyroidism ^[21] — may persist after lithium cessation
CIN patients are particularly vulnerable to acute-on-chronic AKI from volume depletion, NSAIDs, contrast, or new drugs ^[1]
Chronic allograft injury post-transplant: IF/TA is the leading cause of late graft failure ^[10]
Cardiovascular disease is the leading cause of death in CKD patients — accelerated by anaemia, LVH, vascular calcification, and uraemic toxins

Active Recall - Complications of Chronic Interstitial Nephritis

References

^[1] Senior notes: Ryan Ho Urogenital.pdf (Section 4.2 – CIN clinical presentation, concentrating defect, salt wasting, superimposed AKI, analgesic nephropathy) ^[3] Senior notes: Block A - Glomerular and Tubulo-interstitial Diseases and Acute Kidney Injury.pdf (p.28–29 – TIN entities, chronic TIN as pathway to CKD) ^[4] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.333 – Café au lait complexion, CKD manifestations) ^[5] Senior notes: Block A - Chronic Kidney Disease and its Complications.pdf (p.12–13 – CKD clinical manifestations, anaemia, CKD-MBD, drug-induced CKD) ^[6] Senior notes: Block A - Drugs and the Kidney.pdf (p.16 – Aristolochic acid: CIN + urothelial malignancy, prophylactic removal) ^[7] Senior notes: Block A - Nephrotology Teaching Clinic RTD.pdf (p.11 – Aristolochic acid nephropathy: ESRD, urothelial carcinomas) ^[8] Lecture slides: GC 057. Glomerular and Tubulo-interstitial Diseases and Acute Kidney Injury.pdf (p.59 – Entities: AIN, CIN, K-wasting, RTA) ^[9] Senior notes: Block A – Nephrology Data Interpretation.pdf (p.11 – NSAIDs, ACEI/ARB contraindications) ^[10] Senior notes: Block A - Renal Replacement Therapies.pdf (p.40 – Chronic allograft injury: IF/TA, leading cause of late graft failure) ^[11] Senior notes: Block A - Two cases of polyuria and polydipsia.pdf (p.4–5 – Concentrating defect, AQP2, prolonged DI, volume depletion) ^[12] Senior notes: learning_points_output.txt (Renal Tubular Disorders – hypokalaemia perpetuating tubular dysfunction) ^[21] Senior notes: Ryan Ho Psychiatry.pdf (p.53 – Lithium: nephrogenic DI, CIN, hyperparathyroidism) ^[25] Senior notes: Ryan Ho Respiratory.pdf (p.78 – Renal TB: medullary granuloma, chronic TIN, autonephrectomy, ureteral strictures) ^[28] Lecture slides: GC 043. Drugs and the Kidney.pdf (p.16 – Drug-induced TIN may lead to permanent damage)

High Yield Summary

Chronic Interstitial Nephritis — Key Points for Exams:

CIN = chronic inflammatory infiltrate + tubular atrophy + interstitial fibrosis; constitutes 95% of kidney volume affected ^[3]
Major pathway leading to CKD ^[3]; an important but often underrecognised cause
Top aetiologies (HK context): drugs (NSAIDs, PPIs, lithium, calcineurin inhibitors), aristolochic acid (banned in HK since 2004 — causes CIN + urothelial malignancy) ^[6], metabolic (hypercalcaemia, hypokalaemia, hyperuricaemia), obstruction/reflux, autoimmune (Sjögren's, sarcoidosis), infections (TB), inherited (nephronophthisis, cystinosis)
Any untreated AIN can progress to CIN
Clinical "fingerprint" vs glomerular CKD:
- Tubular proteinuria ( < 1.5 g/day), sterile pyuria, no significant haematuria
- Disproportionate anaemia for GFR ^[1]
- Pronounced electrolyte/acid-base disturbance; phosphate may be normal ^[1]
- Early polyuria/nocturia (concentrating defect); late uraemia
- BP normal/low early; hypertension late
Drug-induced CKD classification: Acute ( < 7 days), Subacute (7–90 days), Chronic ( > 90 days) ^[5]
Pathophysiology final common pathway: Tubular injury → inflammation → TGF-β → fibroblast activation → IF/TA → capillary rarefaction → hypoxia → more fibrosis (vicious cycle)
PPIs: acute TIN → chronic TIN → CKD ^[5]; PD-1 inhibitors: acute TIN → AKI ^[5]
NSAIDs: vasoconstriction, AKI, nephrotic syndrome, papillary necrosis, TIN ^[5]

High Yield Summary — Differential Diagnosis of CIN

Compartment-based DDx: Distinguish CIN from chronic GN (heavy proteinuria, active sediment, early HTN), vascular nephropathy (severe HTN, vascular risk factors), obstructive nephropathy (hydronephrosis), and myeloma kidney (dipstick-negative proteinuria, CRAB)
CIN "fingerprint": Tubular proteinuria < 1.5 g/day, sterile pyuria, disproportionate anaemia, pronounced electrolyte disturbance, normal phosphate, normal/low BP early, small kidneys
Key entities within TIN: AIN (drugs, infections, autoimmune), CIN (multiple aetiologies), K⁺-wasting disorders, RTA ^[8]
AIN can progress to CIN if the offending agent is not removed — especially PPIs, NSAIDs
Aetiological DDx within CIN: Drug-induced (analgesics, calcineurin inhibitors, lithium, cisplatin, PPIs), toxins (aristolochic acid, lead), metabolic (nephrocalcinosis, hypokalaemia, hyperuricaemia), autoimmune (Sjögren's, sarcoidosis, SLE, IgG4-RD), structural (reflux, obstruction), inherited (nephronophthisis, cystinosis), infection (TB), idiopathic
In HK: always ask about TCM, OTC NSAIDs, and PPI use

High Yield Summary — Diagnostic Approach to CIN

No formal diagnostic criteria set — CIN is diagnosed by convergence of clinical pattern + urinalysis + bloods + imaging ± histology
Gold standard = renal biopsy showing interstitial fibrosis + tubular atrophy + chronic inflammatory infiltrate (lymphocytes, plasma cells, macrophages) ^[22]
Urinalysis fingerprint: sterile pyuria, WBC casts, tubular proteinuria < 1.5 g/day, no dysmorphic RBCs
Blood fingerprint: NAGMA (RTA), disproportionate NcNc anaemia, normal PO₄, electrolyte disturbance
Imaging fingerprint: bilateral small echogenic kidneys on USG
Urea:Creatinine ratio < 40 → intrinsic renal damage ^[23]
FENa > 1% = tubular damage (cannot reabsorb Na⁺)
Biopsy NOT always needed — can diagnose clinically if clear drug cause, drug is stopped, and patient improves ^[24]
Biopsy IS needed if aetiology unclear, immunosuppression considered, or failure to improve
Do not biopsy very small fibrotic kidneys — high risk, pathologist cannot identify original cause ^[3]
Avoid contrast CT in renal impairment ^[11]

High Yield Summary — Management of CIN

Management is supportive in nature ^[1] — no disease-specific drug for CIN
Most important step: STOP the incriminated drug ^[6]^[28] — this is the mainstay of drug-induced TIN management
Immunosuppression usually NOT necessary, except for: drug-induced vasculitis, immune CPI-associated AKI ^[28], and autoimmune causes (sarcoidosis, SLE, IgG4-RD, Sjögren's)
Steroids only useful if residual active AIN component on biopsy — NOT for established fibrosis
Early steroid treatment within 7 days of drug withdrawal improves outcome; EXCEPT NSAIDs (less responsive) ^[24]
ACEI/ARB for CKD renoprotection ^[30] but cautious in salt-wasting/volume-depleted CIN patients
Electrolyte management is critical — CIN causes pronounced disturbance: oral NaHCO₃ for NAGMA, K⁺ management (direction depends on RTA type), Mg²⁺ replacement
Anaemia management: ESA + iron — anaemia is disproportionate to GFR
Discontinuation of analgesics can only stop progression if done early ^[1]
Lithium: monitor eGFR Q6mo ^[21]; amiloride may reduce lithium entry into collecting duct
Aristolochic acid: stop exposure + screen for urothelial malignancy + consider prophylactic nephroureterectomy ^[7]
Avoid nephrotoxins (NSAIDs, contrast, aminoglycosides); never combine ACEI and ARB ^[9]
Prepare for RRT if approaching stage 5 CKD — dialysis (AEIOU indications) or transplantation

High Yield Summary — Complications of CIN

Tubular dysfunction complications appear EARLY and are DISPROPORTIONATE to GFR — this distinguishes CIN from glomerular CKD
RTA (Types 1, 2, or 4) is one of the four TIN entities ^[3]^[8] — NAGMA with variable K⁺ depending on type
Concentrating defect → polyuria/nocturia → risk of hypernatraemia and volume depletion → superimposed AKI ^[1]
Anaemia: disproportionately severe for GFR ^[1] because EPO-producing interstitial fibroblasts are directly damaged — may need ESA at higher GFR than standard CKD
Phosphate may be NORMAL ^[1] → delays secondary hyperPTH; a distinctive feature of CIN
Aristolochic acid: dual complication of CIN + urothelial malignancy ^[6] — requires lifelong cancer surveillance and possibly prophylactic nephroureterectomy
Analgesic nephropathy: papillary necrosis + CIN + ↑risk of urothelial CA ^[1]
Lithium: nephrogenic DI (70%) + CIN + hyperparathyroidism ^[21] — may persist after lithium cessation
CIN patients are particularly vulnerable to acute-on-chronic AKI from volume depletion, NSAIDs, contrast, or new drugs ^[1]
Chronic allograft injury post-transplant: IF/TA is the leading cause of late graft failure ^[10]
Cardiovascular disease is the leading cause of death in CKD patients — accelerated by anaemia, LVH, vascular calcification, and uraemic toxins

Chronic Interstitial Nephritis

1. Definition

2. Epidemiology

2.1 Prevalence and Burden

2.2 Demographics

2.3 Risk Factors

3. Anatomy and Function of the Tubulointerstitium

3.1 Components

3.2 Functions

3.3 The Medullary Vulnerability

4. Aetiology (with Hong Kong Focus) and Pathophysiology

4.1 Persistent/Untreated Acute Interstitial Nephritis (AIN → CIN)

4.2 Drug-Induced CIN

4.2.1 Analgesic Nephropathy

4.2.2 Calcineurin Inhibitors (Ciclosporin, Tacrolimus)

4.2.3 Lithium

4.2.4 Cisplatin

4.2.5 Tenofovir

4.2.6 PD-1 Inhibitors (Immune Checkpoint Inhibitors)

4.3 Toxin-Induced CIN

4.3.1 Aristolochic Acid Nephropathy (AAN)

4.3.2 Lead Nephropathy

4.3.3 Cadmium Nephropathy

4.4 Metabolic Causes

4.4.1 Nephrocalcinosis (Hypercalcaemia)

4.4.2 Chronic Hypokalaemia

4.4.3 Hyperuricaemia / Uric Acid Nephropathy

4.4.4 Hyperoxaluria

4.5 Structural / Obstructive Causes

4.5.1 Reflux Nephropathy (Chronic Pyelonephritis)

4.5.2 Chronic Urinary Tract Obstruction

4.6 Inherited Causes

4.7 Autoimmune / Systemic Disease

4.7.1 Sjögren's Syndrome

4.7.2 Sarcoidosis

4.7.3 SLE (Lupus Tubulointerstitial Nephritis)

4.7.4 IgG4-Related Disease

4.7.5 Granulomatosis with Polyangiitis (GPA, formerly Wegener's)

4.8 Infection-Related CIN

4.9 Radiation Nephritis

4.10 Other/Idiopathic

5. Classification

5.2 By Pathological Pattern

5.3 By Site of Predominant Damage

5.4 By Clinical Timing

6. Clinical Features

6.1 Symptoms

6.2 Signs

6.3 Characteristic Laboratory Patterns (Clinical Features on Investigations)

6.4 The Tubular Dysfunction Syndromes in CIN

7. Pathophysiology Summary — The Final Common Pathway

Active Recall - Chronic Interstitial Nephritis

References

Framing the Problem: What Are You Actually Differentiating?

Axis 1: Differential Diagnosis by Renal Compartment

The Key Differentials — Compartment by Compartment

1. Chronic Glomerulonephritis (GN)

2. Vascular Nephropathy

3. Obstructive Nephropathy (Post-Renal)

4. Other Important Mimics

a) Multiple Myeloma ("Myeloma Kidney")

b) Polycystic Kidney Disease (PKD)

c) Acute Interstitial Nephritis (AIN)

d) Acute Tubular Necrosis (ATN)

Axis 2: Aetiological Differential Diagnosis Within CIN

Systematic Aetiological Framework

Drug-Induced CIN — Special Considerations for Hong Kong

Lithium-Induced CIN — A Special Differential Worth Knowing

Summary Differential Diagnosis Table

Clinical Approach to Differential Diagnosis

Active Recall - Differential Diagnosis of Chronic Interstitial Nephritis

1. Diagnostic Criteria — Why There Is No Single "Diagnostic Criteria" Set

2. Diagnostic Algorithm

3. Investigation Modalities — Detailed Breakdown

3.1 Urinalysis

3.1.1 Urine Dipstick

3.1.2 Urine Microscopy

3.1.3 Urine Protein Quantification

3.2 Blood Tests — Renal Function and Electrolytes

3.2.1 Renal Function Test (RFT)