A 52-year-old man with a 10-year history of diabetes mellitus type 2 presents with sudden onset gross hematuria, dyspnea, and BP 168/104 mmHg. Urine shows 4+ protein and numerous RBC casts. Serum creatinine is 3.8 mg/dL (baseline 1.0 mg/dL). Complement levels are normal. Anti-GBM antibodies are positive. Chest X-ray shows bilateral pulmonary infiltrates. What is the most likely diagnosis?

Explanation

## Diagnosis: Anti-Glomerular Basement Membrane (Anti-GBM) Disease ### Clinical Presentation This patient presents with **Goodpasture syndrome** — the pulmonary-renal variant of anti-GBM disease: 1. **Rapidly progressive glomerulonephritis (RPGN)** — acute rise in creatinine (3.8 from 1.0 mg/dL), RBC casts, hematuria 2. **Pulmonary hemorrhage** — dyspnea and bilateral infiltrates on CXR 3. **Positive anti-GBM antibodies** — diagnostic serology 4. **Normal complement levels** — distinguishes from immune complex-mediated GN ### Pathophysiology **Key Point:** Anti-GBM disease is a type II hypersensitivity reaction where IgG autoantibodies bind to the NC1 domain of type IV collagen in the basement membrane of the kidneys and lungs. This triggers complement activation and neutrophilic infiltration, causing rapid glomerular destruction. ### Immunofluorescence & Pathology - **Linear IF pattern** — continuous IgG and C3 deposition along the GBM (pathognomonic) - **Crescentic GN** — >50% of glomeruli involved (>80% in Goodpasture syndrome) - **Fibrinoid necrosis** of glomerular capillaries - **Absence of immune deposits** on electron microscopy (pauci-immune) ### Differential: Anti-GBM vs. Other RPGN Causes | Feature | Anti-GBM | ANCA-RPGN | Immune Complex RPGN | |---------|----------|-----------|---------------------| | **IF pattern** | Linear | Pauci-immune | Granular | | **Serology** | Anti-GBM antibody | ANCA (c- or p-) | ANA, anti-dsDNA, low C3 | | **Pulmonary involvement** | Hemorrhage (Goodpasture) | Nodules, infiltrates | Rare | | **Complement** | Normal | Normal | Low | | **Crescent prevalence** | >80% | 50–90% | 50–80% | **High-Yield:** The **linear IF pattern on kidney biopsy** combined with **positive anti-GBM serology** is diagnostic of anti-GBM disease. Pulmonary hemorrhage indicates Goodpasture syndrome (worse prognosis). ### Clinical Features - **Pulmonary-renal syndrome** (Goodpasture) — 40–50% of anti-GBM cases - **Renal-limited disease** — 50–60% of cases (no lung involvement) - **Rapidly progressive renal failure** — 50% reach ESRD within 6 months if untreated - **Peak incidence** — 20–30 and 50–65 years **Clinical Pearl:** Smoking is a risk factor for pulmonary hemorrhage in anti-GBM disease, possibly due to increased alveolar permeability. ### Management 1. **Immediate plasmapheresis** — removes circulating anti-GBM antibodies 2. **Immunosuppression** — corticosteroids + cyclophosphamide (induction therapy) 3. **Prognosis** — depends on baseline creatinine and extent of crescents - Creatinine <5.8 mg/dL at diagnosis: 80% recover renal function - Creatinine >5.8 mg/dL or >90% crescents: poor prognosis **Mnemonic: Goodpasture = Good Prognosis if caught Early** - Early diagnosis and aggressive plasmapheresis improve outcomes - Delay in treatment → irreversible renal failure ### Why Not Diabetic Nephropathy? Diabetic nephropathy is a chronic progressive disease with **normal complement levels** and **granular IF** (if any). The acute presentation with RBC casts, crescents, and positive anti-GBM serology excludes diabetic nephropathy. [cite:Harrison 21e Ch 279; Robbins 10e Ch 20]