The tubulointerstitium is the space between the nephrons (glomeruli + tubules) and the renal vasculature. It consists of:

The interstitium participates in fluid and electrolyte exchange as well as endocrine functions of the kidney ^[6]. It is not just passive scaffolding.

The tubules are embedded in the interstitium. Any interstitial inflammation will:

1. Compress tubules → ↓ tubular flow → oliguria
2. Disrupt peritubular capillary blood supply → tubular ischaemia
3. Impair solute and water reabsorption/secretion → electrolyte disturbances, concentrating defect, acidosis
4. Damage tubular basement membranes → tubular dysfunction (Fanconi syndrome, RTA)
5. Impair erythropoietin production → anaemia (if chronic)

This is overwhelmingly the most common cause. The reaction is typically idiosyncratic (unpredictable, not dose-dependent) and represents a type IV delayed hypersensitivity reaction (T cell–mediated).

Drug-induced AIN is usually an idiosyncratic reaction, occurring 3–5 days after drug intake (but can be several weeks after exposure) ^[2].

Infections can cause AIN through direct invasion of the interstitium or immune-mediated mechanisms ^[2][3].

In Hong Kong, consider leptospirosis (rat urine exposure, especially after typhoons/flooding) and TB (relatively endemic).

Tubulointerstitial nephritis and uveitis (TINU) syndrome: combination of tubulointerstitial nephritis and uveitis, more common in children and young women ^[3].

• Presents with anterior uveitis (eye pain, photophobia, redness) + AKI from TIN
• Systemic symptoms: fever, weight loss, fatigue
• Important to check eyes (slit-lamp exam) in any young patient with unexplained AIN

• No identifiable cause after thorough workup
• May represent an unrecognised drug or autoimmune trigger

Drug-induced AIN is a Type IV (delayed-type) hypersensitivity reaction — this is the same mechanism as contact dermatitis or tuberculin skin test reaction.

Step-by-step mechanism:

1. Hapten formation: The drug (or its metabolite) acts as a hapten — a small molecule that is not immunogenic by itself but becomes so when it binds to a carrier protein (e.g., tubular basement membrane proteins, or Tamm-Horsfall protein).

2. Antigen presentation: The hapten-carrier complex is taken up by antigen-presenting cells (dendritic cells, macrophages) in the renal interstitium and presented to T lymphocytes via MHC molecules.

3. T cell activation: CD4⁺ and CD8⁺ T lymphocytes are activated and infiltrate the interstitium. This is the dominant cell type in the inflammatory infiltrate.

4. Eosinophil recruitment: In many drug-induced cases (up to 40% in NSAID-induced AIN ^[4]), eosinophils are recruited, reflecting the allergic/hypersensitivity nature.

5. Inflammatory cascade: Activated T cells release cytokines (TNF-α, IFN-γ, IL-2, IL-4, IL-5) → further recruitment of inflammatory cells → interstitial oedema → tubular compression and injury.

6. Tubular damage: The inflammatory infiltrate directly damages tubular epithelial cells → tubular dysfunction (electrolyte wasting, concentrating defect, acidosis).

7. If unresolved → fibrosis → chronic interstitial nephritis → CKD.

This explains why AIN is idiosyncratic — it depends on the individual's immune response to the drug-protein complex, not on drug dose. The same drug at the same dose may cause AIN in one patient but not another.

The classic triad (fever + rash + eosinophilia) was originally described in methicillin-induced AIN. Methicillin is a particularly potent hapten that triggers a robust systemic hypersensitivity response. Modern causative drugs (PPIs, NSAIDs) tend to cause a more indolent, localised immune response without prominent systemic manifestations — hence the triad is now rarely seen ^[3].

• Direct mechanism: Organisms invade the interstitium → inflammatory response (e.g., ascending bacterial pyelonephritis)
• Immune-mediated mechanism: Infection triggers an immune response that cross-reacts with interstitial antigens (molecular mimicry) or causes immune complex deposition in the interstitium

• In conditions like Sjögren's syndrome, autoreactive lymphocytes directly infiltrate the renal interstitium (similar to how they infiltrate salivary and lacrimal glands)
• In sarcoidosis, non-caseating granulomas form in the interstitium
• In IgG4-related disease, IgG4⁺ plasma cells and fibrosis dominate the infiltrate

NSAIDs can cause concurrent glomerular disease (minimal change disease or membranous nephropathy) PLUS AIN ^[4]. This is because:

• The T lymphocytes activated in the interstitium also release cytokines that injure podocytes → podocyte foot process effacement → minimal change disease → nephrotic-range proteinuria
• This explains why nephrotic syndrome occurs in < 1% of AIN overall, but is more common in NSAID-induced AIN ^[3]

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

As above: Drugs, Infections, Autoimmune, TINU, Idiopathic.

Entities within tubulointerstitial nephritis ^[1]:

1. Acute interstitial nephritis — drugs, infections, autoimmune disease
2. Chronic interstitial nephritis — etiology often not well known
3. K⁺-wasting tubular disorders
4. Renal tubular acidosis

AIN is classified as an intrinsic renal cause of AKI (specifically tubulointerstitial) ^[9]:

• Pre-renal → hypovolaemia, shock
• Intrinsic renal → glomerular (GN), tubulointerstitial (ATN, AIN), vascular
• Post-renal → obstruction

AIN is mostly clinically silent ^[2]. The presentation depends on:

1. The severity of the interstitial inflammation
2. The cause (drug vs infection vs autoimmune)
3. Whether there is a systemic hypersensitivity response

Classic triad of drug hypersensitivity (seen in only ~10% of patients): Fever + Eosinophilia + Skin rash ± Arthralgia ^[2][3][6]

Not commonly seen nowadays due to the absence of cases of methicillin-induced AIN in which this triad originates from ^[3]

Why is the triad rare now?

• Methicillin is essentially never used clinically anymore
• Modern causative drugs (PPIs, NSAIDs, checkpoint inhibitors) tend to produce a more indolent, organ-limited immune response without robust systemic hypersensitivity

These are not strictly "clinical features" in the traditional sense, but are often the first clue to AIN:

Graves' disease can be associated with autoimmune interstitial nephritis, especially in the context of Sjögren's syndrome ^[8]

• Sjögren's → lymphocytic infiltration of renal interstitium → distal RTA (Type 1) and nephrogenic DI
• Chronic interstitial nephritis with positive rheumatoid factor and positive anti-Ro → Sjögren syndrome ^[8]
• May present with polyuria, polydipsia (nephrogenic DI), hypokalaemia, non-AG metabolic acidosis

• Increasingly recognised cause of AIN with dense storiform fibrosis and IgG4⁺ plasma cell infiltrate
• Often multi-organ involvement (autoimmune pancreatitis, sclerosing cholangitis, sialadenitis)
• Responds to corticosteroids

• Anti-PD-1/PD-L1 and anti-CTLA-4 agents → removal of immune "brakes" → T cells attack self-antigens including renal interstitial antigens
• Immune checkpoint inhibitor–associated AKI requires immunosuppression (corticosteroids), unlike most drug-induced AIN where simply stopping the drug suffices ^[5]

• Allopurinol may cause SJS/TEN, acute hepatitis, acute TIN ^[5]
• HLA-B5801* allele (prevalent in Chinese/Southeast Asian populations) confers high risk
• Pre-prescription HLA testing is now standard of care before starting allopurinol in Chinese patients

Before diagnosing AIN, you first need to confirm that the patient has AKI. The universally accepted framework is the KDIGO 2012 criteria ^[15][16]:

AKI is diagnosed if ANY ONE of the following is present ^[16]:

• ↑ Serum creatinine ≥ 26.5 μmol/L within 48 hours
• ↑ Serum creatinine to ≥ 1.5× baseline within 7 days
• Urine output < 0.5 mL/kg/h for ≥ 6 hours

KDIGO AKI Staging ^[16]:

In Hong Kong we use μmol/L for creatinine, whereas other parts of the world use mg/dL ^[15]. (Conversion: 1 mg/dL ≈ 88.4 μmol/L)

Once AKI is confirmed, you must determine where the injury is:

AKI aetiology is classically separated into ^[1][9]:

• Pre-renal ( > 1/2): ineffective perfusion
• Intrinsic renal ( < 1/2): intrinsic renal disease
  • Tubular necrosis (ischaemia, toxins)
  • Interstitial nephritis
  • Acute GN / RPGN
• Post-renal ( < 10%): obstructive uropathy

The diagnostic workup systematically excludes pre-renal and post-renal causes first, then differentiates between the intrinsic renal compartments.

A clinical (presumptive) diagnosis of AIN can be made when the following constellation is present:

Renal biopsy is usually required to confirm the diagnosis ^[6].

However:

Renal biopsy is the definitive diagnosis but is often NOT necessary ^[3]:

• NOT needed if diagnosis is likely, if the presumed precipitating drug can be easily withdrawn, and if the patient improves after withdrawal of a potentially offending drug
• Indicated when diagnosis is unclear (non-characteristic urinalysis), if patient is not improving, or if glucocorticoid treatment is being considered ^[3]

Drug-induced AIN is a Type IV hypersensitivity reaction. The drug (acting as a hapten) is the ongoing antigenic stimulus. Remove the antigen → the immune response winds down → inflammation resolves → tubular and interstitial recovery begins.

Ensure and manage ABC: "A dead person has no renal function" ^[9]

Avoid nephrotoxins, e.g. NSAIDs, potential nephrotoxins ^[2]

Temporary dialysis if necessary ^[2]

Dialysis is the last resort when conservative medical management fails. Use the AEIOU mnemonic ^[9]:

Most patients with AIN do not require dialysis. Those who do typically have severe AIN with prolonged oliguria or dialysis-dependent AKI at presentation.

AIN is an immune-mediated inflammatory process (Type IV hypersensitivity with T cell activation). Glucocorticoids:

1. Suppress T cell activation (reduce IL-2 production, inhibit T cell proliferation)
2. Reduce cytokine release (TNF-α, IFN-γ, IL-1, IL-6) → less inflammatory infiltrate
3. Reduce interstitial oedema → less tubular compression → faster GFR recovery
4. Prevent long-term scarring — by stopping the inflammation before it progresses to fibrosis ^[2]

Immunosuppression: usually not necessary, except for severe cases / systemic manifestations ^[5]:

• Drug-induced vasculitis
• Drug-induced muco-cutaneous manifestations
• Immune checkpoint inhibitor–associated AKI
• Drug-induced hepatitis

Immunosuppression: usually require confirmation by renal biopsy ^[2]

Specific indications:

Glucocorticoids: ? accelerates recovery, prevents long-term scarring ^[2]

Early steroid treatment within 7 days of drug withdrawal improves outcome in drug-induced AIN ^[3]

This is a critical teaching point: timing matters. The longer you wait, the more likely the inflammation has progressed to fibrosis, which steroids cannot reverse.

Steroids are indicated in all patients with suspected drug-induced AIN EXCEPT NSAIDs, which is less likely to respond to steroids ^[3]

Efficacy: controversial, ? less efficacious in NSAID-related AIN ^[2]

Why might NSAIDs respond less to steroids?

The mechanism of NSAID-induced AIN may involve additional non-immune pathways (direct prostaglandin inhibition → haemodynamic component) alongside the immune-mediated component. The concurrent minimal change disease (which itself IS steroid-responsive in primary MCD) adds complexity — but in NSAID-MCD, the underlying trigger is the drug, and removing the NSAID is the primary treatment.

This is an increasingly important clinical scenario as immunotherapy use expands in oncology.

Immune checkpoint inhibitor–associated AKI: most of the time this side effect is an off-target effect, due to the excessive stimulation of the immune system ^[5]

Responsive to steroids → those not given steroids did not improve ^[5]

For steroid-refractory or steroid-dependent AIN, limited data support:

• CXR may show pulmonary oedema (bat-wing appearance) ^[20]
• Severe pulmonary oedema is a life-threatening complication requiring urgent IV loop diuretics (furosemide) or dialysis if refractory ^[9]

• Symptoms usually only when K⁺ > 6 mmol/L: arrhythmia, weakness ^[9]
• ECG changes: peaked T waves → flattened P waves → widened QRS → sine wave → cardiac arrest
• Requires urgent treatment: IV calcium gluconate (cardioprotection), NaHCO₃ infusion, dextrose/insulin drip, PO polystyrene sulfonate, dialysis ^[9]

• Renal tubular acidosis is one of the four clinical entities within tubulointerstitial nephritis ^[1][19]
• Severe acidosis (pH < 7.1) that is refractory to IV bicarbonate is an indication for dialysis ^[9]

When GFR drops sufficiently, nitrogenous waste products (urea, creatinine, and other "uraemic toxins") accumulate:

• Features of uraemia (pericarditis, neuropathy, ↓ mental status) are indications for dialysis ^[9]

• Patients with AIN are immunocompromised at multiple levels:
  1. The AKI itself impairs immune function (uraemia → leukocyte dysfunction)
  2. If treated with glucocorticoids → further immunosuppression
  3. If on dialysis → catheter-related bloodstream infections
• In particular, patients with drug hypersensitivity–associated AIN who are also febrile must be carefully evaluated to distinguish the fever of hypersensitivity from a superimposed infection

NSAID-induced nephrotic syndrome + AKI: minimal change glomerulopathy + acute tubulointerstitial nephritis, with T lymphocytes and eosinophils (40%), higher risk in elderly ^[4]

The concurrent nephrotic syndrome introduces additional complications:

• Severe oedema / anasarca — from massive proteinuria → hypoalbuminaemia → ↓ oncotic pressure
• Thromboembolism — nephrotic syndrome is a hypercoagulable state (loss of antithrombin III, protein S, protein C in urine; increased hepatic synthesis of clotting factors)
• Increased infection risk — urinary loss of immunoglobulins
• Hyperlipidaemia — hepatic response to hypoalbuminaemia → increased VLDL and LDL synthesis

This is the single most important long-term complication of AIN.

Even mild, reversible AKI has important clinical consequences, e.g. CKD transition ^[1]

AKI may lead to permanent damage ^[4]

Chronic TIN is the major pathway leading to chronic kidney disease ^[19]

Why does AIN lead to CKD?

The pathophysiological sequence is:

1. Persistent interstitial inflammation (if the drug is not stopped, or the autoimmune process is not controlled)
2. Fibroblast activation → deposition of collagen and extracellular matrix in the interstitium
3. Interstitial fibrosis → replaces normal interstitial tissue with scar tissue
4. Tubular atrophy → chronically compressed and damaged tubules lose their epithelial lining and function
5. Loss of peritubular capillaries → chronic ischaemia → further fibrosis (a vicious cycle)
6. Nephron loss → ↓ functioning nephron mass → ↑ workload on remaining nephrons → hyperfiltration → secondary glomerulosclerosis → further nephron loss

Classification of chronic TIN on histology ^[6]:

• Interstitial inflammation — lymphocytes + plasma cells + macrophages
• Interstitial fibrosis
• Tubulitis with focal tubular atrophy

This is in contrast to acute TIN histology which shows interstitial oedema (reversible) rather than fibrosis (irreversible).

PPIs cause acute tubulointerstitial nephritis → AKI, and chronic tubulointerstitial nephritis → CKD ^[7]

If AIN progresses to CKD, the patient develops all the systemic manifestations of CKD ^[7]:

Even if GFR partially recovers, residual tubular damage can cause chronic tubular dysfunction:

CIN clinical presentation: concentration defect (early) → polyuria/nocturia ± salt wasting (↓ BP, hypovolaemic hypoNa). Due to tubular damage to medullary area. May have superimposed AKI due to inadequate perfusion ^[19]

Prognosis: not well-described; mean recovery time 1.5 months but may be incomplete ^[2]

• Not all patients return to their baseline creatinine
• Risk factors for incomplete recovery:
  • Prolonged drug exposure before cessation
  • Delayed recognition and treatment
  • Significant interstitial fibrosis on biopsy
  • Pre-existing CKD (less renal reserve)
  • NSAID-induced AIN (less responsive to steroids)
  • Older age
• Patients with incomplete recovery carry residual CKD and are at risk for all CKD complications

Chinese herbal medicine and tubulointerstitial nephritis: some traditional Chinese herbal preparations contain the toxin aristolochic acid ^[6]

Aristolochic acid nephropathy: marked interstitial fibrosis and tubular atrophy, usually few interstitial inflammatory cells. Progression to end-stage renal failure within months ^[6]

This is a particularly aggressive form of chronic TIN that:

• Progresses rapidly to ESRD requiring dialysis
• Has an increased risk of urothelial carcinomas (transitional cell carcinoma of the bladder, ureter, renal pelvis) ^[6] — aristolochic acid is a potent mutagen that forms DNA adducts
• The implicated herbs are banned in Hong Kong (關木通, 廣防己, 馬兜鈴, 青木香, 天仙藤) ^[6]

If steroids are used (prednisolone 1 mg/kg/day for 2–3 weeks, tapered over 3–4 weeks), the short course limits long-term steroid toxicity but can still cause:

If temporary dialysis is required: