• Rare disease: incidence approximately 1.64 per million per year ^[2]
• Accounts for < 5% of all glomerulonephritis but 15–20% of all RPGN ^[2]
• Accounts for ~1–5% of all crescentic glomerulonephritis cases globally
• More common in Caucasian populations; relatively less common but still reported in East Asian (including Hong Kong Chinese) populations

• Bimodal age distribution:
  • First peak: Young adults (20–30 years), predominantly male, more likely to have full-blown Goodpasture's syndrome (pulmonary-renal involvement) ^[2]
  • Second peak: Older adults ( > 50–60 years), slight female predominance, more likely to have isolated renal disease without lung involvement ^[2]
• Overall slight male predominance (~60% male)

"Younger more likely to be full-blown Goodpasture's but older patients ( > 50y) more likely to have isolated GN" ^[2]

• Anti-GBM disease is rare in Hong Kong but is a recognised cause of RPGN
• In Hong Kong, the most common causes of RPGN are ANCA-associated vasculitis and IgA nephropathy with crescents, but anti-GBM disease must always be considered given its rapidly fulminant course
• HLA associations (HLA-DRB1*1501) are present across ethnic groups including Chinese populations

"Genetic susceptibility: a/w HLA-DRB1*1501/1502" ^[2]

Most patients have no identifiable trigger ^[2]. However, when triggers are identified:

"Those with lung involvement more commonly a/w underlying pulmonary injury, eg. smoking, infection ('exposes' the pulmonary Ag)" ^[2]

• Post-transplant anti-GBM disease can occur in 5–10% of renal transplant recipients with underlying Alport syndrome ^[2]
• Why? Alport syndrome = mutations in COL4A3/A4/A5 genes → abnormal or absent α3/α4/α5(IV) collagen chains. When these patients receive a normal kidney transplant, they are exposed to normal α3(IV) collagen for the first time. Their immune system has never been tolerised to this antigen → de novo anti-GBM antibody production against the transplanted kidney
• This is called post-transplant anti-GBM nephritis (a form of alloimmune anti-GBM disease)

The glomerular filtration barrier consists of three layers (from capillary lumen to Bowman's space):

Capillary Lumen
       ↓
1. Fenestrated endothelium (70-100 nm pores)
       ↓
2. Glomerular Basement Membrane (GBM) — the TARGET
       ↓
3. Podocyte foot processes with slit diaphragms
       ↓
Bowman's Space (→ proximal tubule)

The GBM is a specialised extracellular matrix composed of:

• Type IV collagen (the structural scaffold) — forms a network of triple-helical protomers
• Laminin (cell adhesion)
• Nidogen/entactin (cross-linking)
• Heparan sulphate proteoglycans (charge barrier — negative charge repels albumin, which is also negatively charged)

Type IV collagen has 6 α chains (α1 through α6). The mature GBM contains:

• α3(IV), α4(IV), and α5(IV) collagen chains assembled into a heterotrimer: α3α4α5(IV)

The NC1 (non-collagenous 1) domain is the globular domain at the C-terminal end of each collagen IV chain. The α3(IV) NC1 domain is the Goodpasture antigen — the specific epitope targeted by anti-GBM antibodies.

The α3(IV) collagen chain is found in:

Why is the kidney always affected but the lung only sometimes?

• In the kidney, the GBM antigen is constitutively exposed to the circulation (the glomerular capillary is fenestrated → antibodies have direct access)
• In the lung, the alveolar basement membrane is protected by intact endothelium → antibodies cannot access the antigen unless the endothelium is damaged (by smoking, infection, etc.)

Similar architecture to the GBM but thinner. It contains the same α3α4α5(IV) collagen network. The alveolar-capillary membrane normally acts as a barrier preventing antibody access to the basement membrane collagen. Injury (smoking, infection, fluid overload, high FiO₂) disrupts this barrier.

• Target: NC1 domain of α3 chain of type IV collagen (the Goodpasture antigen)
• Antibody class: Predominantly IgG (IgG1 and IgG3 subclasses → potent complement activation), rarely IgA or IgM
• Specificity: The antibodies bind to two specific epitopes (EA and EB) on the α3(IV) NC1 domain. These epitopes are normally cryptic — hidden within the hexameric structure of collagen IV and only become accessible when the quaternary structure is disrupted

Detailed mechanistic narrative:

1. Initiation — Loss of tolerance: In genetically susceptible individuals (HLA-DRB1*1501), an unknown trigger (possibly molecular mimicry, infection, or environmental exposure) breaks immune tolerance to the α3(IV) NC1 domain. CD4+ T helper cells recognise the Goodpasture antigen presented by HLA class II molecules → activate autoreactive B cells

2. Autoantibody production: B cells differentiate into plasma cells producing high-affinity IgG anti-GBM antibodies. These antibodies are transiently produced — the autoimmune response is typically self-limited (lasting weeks to months), which is a key feature distinguishing anti-GBM disease from other autoimmune conditions. This has therapeutic implications (plasma exchange can remove the antibodies, and they may not recur)

3. Antibody binding: Anti-GBM antibodies bind uniformly and continuously along the entire length of the GBM → this creates the characteristic linear IgG staining pattern on immunofluorescence ^[1][2][3]. This is in contrast to:

   • Granular pattern (immune complex deposition, e.g., lupus, IgAN)
   • Negative/pauci-immune pattern (ANCA vasculitis)

4. Complement activation: IgG1/IgG3 binding activates the classical complement pathway → C3a/C5a (anaphylatoxins) recruit neutrophils and monocytes; C5b-9 (membrane attack complex) causes direct GBM injury

5. Inflammatory cell recruitment: Neutrophils and macrophages accumulate in the glomerular capillaries → release reactive oxygen species, proteases, and cytokines → further GBM damage

6. GBM breaks and crescent formation: Severe GBM damage creates gaps ("breaks") → plasma proteins (including fibrinogen/fibrin) and inflammatory cells leak into Bowman's space → "massive movement of plasma product into Bowman's space → subsequent massive influx of MQs/T cells, release of proinflammatory cytokines → formation of cellular crescent" ^[1][4]

7. Crescent evolution:

   • Cellular crescents (proliferating parietal epithelial cells + macrophages + fibrin) — potentially reversible with aggressive immunosuppression
   • Fibrocellular crescents — partly reversible
   • "Fibrous crescents" — "unlikely to respond to immunosuppressive treatment" ^[1][4] — irreversible

8. Progression to ESRD: Without treatment, > 90% progress to end-stage renal disease or death ^[2]

"Pathogenesis: transient production of circulating autoAb vs GBM (IgG) against the Goodpasture Ag — NC1 domain of α-3 chain of collagen IV" ^[2]

"Autoreactive Ab → potent inflammatory response → crescent formation with glomerular destruction" ^[2]

• Anti-GBM antibodies also bind the alveolar basement membrane (same α3(IV) collagen)
• Pulmonary capillaritis → damage to alveolar capillaries → red blood cells leak into the alveolar spaces → diffuse alveolar haemorrhage (DAH)
• Haemoglobin in the alveoli is taken up by alveolar macrophages → haemosiderin-laden macrophages (siderophages)
• Chronic/recurrent pulmonary haemorrhage → iron-deficiency anaemia (iron is sequestered in the lungs)

• ANCA may be present in 10–50% of anti-GBM patients ^[2]
• Usually MPO-ANCA (p-ANCA) positive
• Clinical significance: dual-positive patients may have a slightly better prognosis because ANCA-associated vasculitis tends to respond better to immunosuppression and has a more relapsing-remitting course
• These patients may have features of both anti-GBM disease and ANCA vasculitis
• It is hypothesised that ANCA-mediated vasculitis may precede and trigger anti-GBM disease by disrupting the GBM and exposing the Goodpasture antigen

"ANCA: may be present in 10-50% anti-GBM pt, indicates better response to Tx" ^[2]

RPGN is classified based on immunofluorescence (IF) staining pattern ^[1][3][4][5]:

"Type I (linear staining) due to anti-GBM disease" ^[1][4]

"Classified based on IF staining pattern" ^[3][4]

Under the KDIGO framework, anti-GBM disease is classified as:

• Immune complex and complement-mediated GN → specifically under "Anti-GBM nephritis"
• Distinguished from immune complex GN (e.g., lupus nephritis, IgAN) and pauci-immune GN (ANCA-associated)

Anti-GBM disease presents as RPGN ± pulmonary haemorrhage ^[2][3].

The presentation is typically:

• Acute or subacute onset (days to weeks)
• "Abrupt onset of S/S of GN, eg. haematuria, proteinuria, RBC casts" ^[4]
• "Rapidly declining renal function leading to ESRD in days to weeks" ^[4]
• "Often preceded by insidious onset of fatigue or oedema" ^[4]
• A preceding flu-like prodrome or upper respiratory tract infection may be present (but is not necessary)

This is our index condition. Key distinguishing features:

ANCA vasculitis is the most common cause of RPGN overall and is the single most important differential diagnosis for anti-GBM disease. It can also present with pulmonary-renal syndrome, making it clinically indistinguishable from anti-GBM disease without serology.

"Palpable purpura/petechial rash suggest vasculitis" ^[4][6]

"Triad of sinusitis, pulmonary infiltrates and nephritis suggest Wegener's" ^[4][6]

Why this matters: Both ANCA vasculitis and anti-GBM disease can cause RPGN + pulmonary haemorrhage. The key clinical distinguisher is that ANCA vasculitis typically has extra-renal/extra-pulmonary systemic features (rash, neuropathy, sinusitis, arthralgia) while anti-GBM disease is organ-specific (kidney ± lung only, no rash, no sinusitis). However, 10–50% of anti-GBM patients are double-positive (anti-GBM + ANCA), making the picture even more complex — always send both ANCA and anti-GBM together.

The diagnosis of anti-GBM disease requires at least one of the following two (ideally both):

In clinical practice, you typically have both — positive serology AND confirmatory biopsy. However:

• If serology is strongly positive and the patient is too sick for biopsy (e.g., on ventilator with DAH, coagulopathy), treatment may be started on serology alone
• If serology is negative but clinical suspicion is very high, biopsy may reveal the diagnosis (rare "seronegative" anti-GBM disease — antibodies present in tissue but below detection threshold in serum)

"Practice Point 1.1.1: The kidney biopsy is the 'gold standard' for the diagnostic evaluation of glomerular diseases. However, under some circumstances, treatment may proceed without a kidney biopsy confirmation of diagnosis" — KDIGO GN Guideline 2021 ^[5]

The diagnostic workup follows a parallel, time-critical approach — investigations are sent simultaneously, not sequentially.

1. Never wait for serology before starting treatment if clinical suspicion is high: "Can give empirical pulse IV methylprednisolone before renal Bx if indicated" ^[1][4]. If a patient presents with RPGN + DAH, give pulse methylprednisolone and arrange plasma exchange while serology is pending.

2. Send anti-GBM AND ANCA together: Because 10–50% of patients are double-positive, and the distinction matters for treatment and prognosis ^[2].

3. Complement levels narrow the differential before serology returns: Normal complement → anti-GBM or ANCA vasculitis (treat aggressively). Low complement → immune complex GN (different treatment pathway) ^[1][4][6].

4. Biopsy as soon as safely feasible: Even with positive serology, biopsy provides:

   • Prognostic information (% crescents, cellular vs fibrous, degree of sclerosis)
   • Confirmation (rules out false positive serology)
   • Guides treatment intensity (fibrous crescents = unlikely to respond to immunosuppression) ^[1][4]

5. Do NOT biopsy if kidneys are small (chronic irreversible disease) — "Renal biopsy: necessary for most cases of nephritic syndrome unless very small kidney on USG" ^[4][6]

"Urinalysis: document glomerular haematuria, proteinuria and sterile pyuria" ^[4][6]

"All Hb-positive dipstick should be accompanied by urine microscopy to differentiate haematuria vs pigmenturia" ^[12]

Why dysmorphic RBCs? Red blood cells that pass through the damaged GBM are mechanically distorted by squeezing through small breaks and gaps → they lose their normal biconcave disc shape and develop irregular protrusions ("acanthocytes" with Mickey-Mouse ear projections). They are then further distorted by osmotic changes as they travel through tubules with varying tonicity. By contrast, isomorphic (normal-shaped) RBCs suggest a lower urinary tract source (bladder, prostate, urethra) where RBCs enter the urine without passing through a filtration barrier ^[12].

Why RBC casts are pathognomonic of glomerular bleeding? RBCs leaking from damaged glomeruli travel down the tubule, where they become embedded in Tamm-Horsfall glycoprotein (secreted by tubular epithelial cells). This glycoprotein matrix gels together forming a cylindrical "cast" of the tubule shape, trapping the RBCs inside. This can only happen if the RBCs originated from the glomerulus — RBCs from the bladder or lower tract never enter the tubular system.

"Preliminary tests: CBC: NcNc anaemia with ↓Hct ± ↑WBC (if recent infection); RFT: document degree of renal impairment; ESR: usually increased" ^[1][4][6]

This is the most important set of tests. All should be sent simultaneously and urgently (ideally with a verbal "urgent" request to the lab).

"Serology for relevant conditions: ANCA and its subtypes for ANCA-vasculitis; ANA, anti-dsDNA for lupus nephritis; Anti-GBM autoAb for anti-GBM disease; Anti-streptolysin O (ASLO) for PSGN" ^[4][6]

Workup: "CBC, RFT, urinalysis; Complement; Serology: ANCA, anti-GBM, ANA, anti-dsDNA; CXR, DLCO for pulmonary involvement; ± renal biopsy" ^[1][4]

Understanding the Anti-GBM ELISA:

• ELISA = Enzyme-Linked ImmunoSorbent Assay
  • "Immuno" = uses antibodies; "Sorbent" = bound to a solid surface; "Enzyme-Linked" = enzyme conjugate produces a colour change proportional to antibody concentration
• The assay plate is coated with recombinant NC1 domain of α3(IV) collagen (the Goodpasture antigen)
• Patient serum is added → if anti-GBM IgG is present, it binds to the antigen
• An enzyme-linked secondary anti-human IgG antibody is added → colour change measured by spectrophotometry
• Quantitative result: antibody titre correlates with disease activity and can be used to monitor treatment response
• Serial monitoring: anti-GBM titres should fall with plasma exchange and immunosuppression; persistent elevation suggests ongoing antibody production

"CXR, DLCO for pulmonary involvement" ^[1][4]

Why CXR infiltrates in DAH spare the apices and costophrenic angles? The alveolar haemorrhage follows a gravitational distribution → blood preferentially pools in the dependent (lower and central) lung zones. The apices and extreme bases are relatively spared because capillary hydrostatic pressure and blood flow are lower in these regions.

When pulmonary involvement is suspected:

What it is: A procedure that physically separates the patient's plasma (containing the pathogenic anti-GBM antibodies) from the blood cells, discards the plasma, and replaces it with albumin solution or fresh frozen plasma (FFP).

• "Plasma" = the liquid portion of blood containing proteins, antibodies, clotting factors
• "Pheresis" (from Greek ἀφαίρεσις) = removal, taking away
• So plasmapheresis = "taking away the plasma"

How it works mechanistically:

1. Blood is drawn from the patient via a large-bore venous catheter (usually a Quinton catheter in the internal jugular or femoral vein)
2. Blood passes through a centrifuge or membrane filter that separates plasma from blood cells
3. The plasma (containing anti-GBM IgG) is discarded
4. Blood cells are returned to the patient mixed with replacement fluid (5% albumin ± FFP)
5. Each session removes approximately 60% of circulating IgG
6. Over 2–3 weeks of daily/alternate-day sessions, the antibody titre drops dramatically

Indications:

• All patients with anti-GBM disease who have pulmonary haemorrhage ^[2]
• Non-dialysis-dependent renal involvement ^[2]
• Critical for antibody removal — the faster you remove antibodies, the less GBM destruction occurs

Contraindications/Cautions:

Complications of plasma exchange:

"Plasma exchange to remove circulating antibodies — Regimen: daily or alternate-day 4 litre exchanges × 2-3 weeks + albumin/FFP as replacement fluid" ^[2]

Purpose: Suppress the inflammatory cascade triggered by antibodies already deposited in the GBM. Even after plasma exchange removes circulating antibodies, the antibodies already bound to the GBM continue to activate complement and recruit inflammatory cells → corticosteroids dampen this ongoing inflammation.

Phase 1 — Induction (Pulse IV Methylprednisolone):

"IV pulse MP × 3 days" ^[3]

Phase 2 — Maintenance (Oral Prednisolone):

"Followed by oral prednisolone (tapered after remission)" ^[2]

Side effects of corticosteroids (important for exam — common SAQ topic):

Prophylaxis during steroid therapy:

• PPI (omeprazole/pantoprazole) for gastroprotection
• Calcium + Vitamin D ± bisphosphonate for osteoporosis prevention
• PJP prophylaxis (co-trimoxazole) when on high-dose steroids + cyclophosphamide
• Blood glucose monitoring (steroid-induced diabetes)

Purpose: Suppress ongoing autoantibody production by killing rapidly dividing immune cells (particularly B lymphocytes that differentiate into antibody-producing plasma cells).

• "Cyclo" = cyclic; "phosph" = phosphorus-containing; "amide" = nitrogen-containing group
• It is a nitrogen mustard alkylating agent — cross-links DNA strands, preventing cell division
• Works best against rapidly dividing cells → lymphocytes, particularly B cells

"Oral cyclophosphamide to suppress further antibody synthesis" ^[2]

"± cyclophosphamide / IV rituximab" ^[3]

Indications:

• All patients receiving treatment for anti-GBM disease (standard component of triple therapy)

Contraindications:

Side effects:

Purpose: Selectively depletes CD20+ B lymphocytes → eliminates the cells that differentiate into antibody-producing plasma cells. Increasingly used as an alternative to cyclophosphamide, particularly in younger patients (avoids gonadal toxicity) and those intolerant to cyclophosphamide.

• "Ri-tuxi-mab": "-mab" = monoclonal antibody; "tu" = tumour-targeted (originally developed for B-cell lymphoma); "xi" = chimeric (mouse/human)
• It is a chimeric anti-CD20 monoclonal antibody

"± cyclophosphamide / IV rituximab" ^[3]

Indications for rituximab over cyclophosphamide:

• Young patients of reproductive age (fertility preservation)
• Previous cyclophosphamide toxicity or intolerance
• Double-positive disease (anti-GBM + ANCA) — the ANCA component may relapse, and rituximab is effective for ANCA maintenance
• Some centres now use rituximab as first-line instead of cyclophosphamide based on ANCA vasculitis trial data (RAVE, RITUXVAS), though specific anti-GBM RCT data is limited

Contraindications:

Side effects:

ESRD is the single most important complication and the defining adverse outcome of anti-GBM disease.

Why is the renal prognosis so poor? Several factors conspire:

1. Anti-GBM disease presents explosively — by the time the patient is symptomatic (haematuria, oliguria, uraemia), extensive crescentic destruction has already occurred
2. The damage accelerates: each destroyed glomerulus shifts the filtration burden to remaining glomeruli → hyperfiltration injury → secondary FSGS and tubulointerstitial fibrosis → further nephron loss (the "common pathway" to ESRD that applies to all renal pathologies)
3. Once crescents become fibrous, they are irreversible regardless of treatment intensity

Consequences of ESRD:

DAH is the complication that can kill the patient acutely, unlike ESRD which is manageable with dialysis.

Complications of DAH itself:

The rapid loss of GFR in RPGN produces several life-threatening metabolic emergencies ^[15]:

"Management of life-threatening complications: Fluid overload — Mx: IV loop diuretics, dialysis; Hyperkalaemia — Mx: IV Ca, NaHCO₃ infusion, dextrose/insulin drip, PO polysulphonate, dialysis; Metabolic acidosis — Mx: IV bicarbonate, dialysis" ^[15]

Indications for dialysis (AEIOU): Acidosis, Electrolyte imbalance (hyperK), Intoxication, Overload (fluid), Uraemia ^[15]

"Hypercoagulability by compensatory production of clotting factors by liver" → "Doppler USG, CT angiography; if AKI: thrombolysis ± embolectomy; if non-AKI: LMWH/UFH → warfarin for minimum 6–12 months while still nephrotic" ^[16]

Prophylaxis:

• PPI for gastroprotection
• Calcium + vitamin D ± bisphosphonate for bone protection
• Co-trimoxazole for PJP prophylaxis (when on cyclophosphamide + high-dose steroids)
• Blood glucose monitoring