A 58-year-old male presents with a 3-week history of progressive dyspnea, chest pain, and hemoptysis. On examination, he has a blood pressure of 165/95 mmHg, heart rate 102/min, and bilateral crackles on auscultation. Laboratory investigations reveal: serum creatinine 3.2 mg/dL (baseline 0.9), urinalysis shows RBC casts and proteinuria (3.5 g/24 h), and chest X-ray shows bilateral alveolar infiltrates. ANCA testing is negative. Chest CT angiography rules out pulmonary embolism. What is the most likely diagnosis?

Explanation

## Clinical Presentation & Diagnosis This patient presents with the classic triad of **pulmonary-renal syndrome**: hemoptysis, dyspnea with bilateral infiltrates, and rapidly progressive glomerulonephritis (RPGN) with RBC casts. ### Key Diagnostic Features of Anti-GBM Disease (Goodpasture Syndrome) **Key Point:** Anti-GBM disease presents with pulmonary hemorrhage + RPGN, typically with NEGATIVE ANCA serology — a critical distinguishing feature. **High-Yield:** The combination of: - Hemoptysis + bilateral alveolar infiltrates (pulmonary hemorrhage) - RBC casts + proteinuria + acute renal failure (RPGN) - **ANCA-negative** serology This triad is pathognomonic for anti-GBM disease. ### Pathophysiology Anti-GBM disease is caused by IgG autoantibodies directed against the NC1 domain of type IV collagen in basement membranes of the lungs and kidneys. The antibodies bind in situ, activate complement, and cause linear IgG deposition on immunofluorescence (IF) — a hallmark finding. ### Diagnostic Confirmation | Investigation | Anti-GBM Disease | GPA | MPA | | --- | --- | --- | --- | | ANCA | **Negative** | Positive (c-ANCA, PR3) | Positive (p-ANCA, MPO) | | Serum Anti-GBM | **Positive** | Negative | Negative | | Kidney IF | **Linear IgG** | Granular (ANCA+) | Granular (ANCA+) | | Pulmonary involvement | Common (60–90%) | Very common | Less common | | Upper respiratory tract | Absent | **Classic** (sinusitis, granulomas) | Absent | **Clinical Pearl:** Anti-GBM disease is the ONLY pulmonary-renal vasculitis that is ANCA-negative. This negative ANCA in the setting of pulmonary hemorrhage + RPGN should immediately raise suspicion for anti-GBM disease. ### Why This Patient Has Anti-GBM and Not ANCA-Associated Vasculitis 1. **ANCA is negative** — rules out GPA and MPA 2. **Absence of upper respiratory tract involvement** — GPA typically presents with sinusitis, nasal granulomas, or otitis media 3. **Acute presentation with severe pulmonary hemorrhage** — anti-GBM disease often presents more acutely and fulminantly than MPA 4. **Linear IF pattern** (if kidney biopsy were performed) — would confirm anti-GBM disease **Mnemonic:** **ANCA-Negative Pulmonary-Renal = Anti-GBM** — when you see hemoptysis + RPGN + negative ANCA, think Goodpasture first. ### Management Anti-GBM disease is a medical emergency requiring: 1. Plasmapheresis (to remove circulating anti-GBM antibodies) 2. Immunosuppression (corticosteroids + cyclophosphamide) 3. Early intervention is critical — prognosis depends on baseline creatinine and percentage of crescents on biopsy [cite:Robbins 10e Ch 20]