A 32-year-old woman of Indian origin presents with a 1-week history of gross hematuria, facial puffiness, and dyspnea on exertion. Blood pressure is 162/104 mmHg. She denies recent infections or rashes. Urinalysis shows 4+ blood, RBC casts, and 2.5 g/day proteinuria. Serum creatinine is 2.1 mg/dL (baseline unknown). C3 and C4 levels are normal. ANA and anti-GBM antibodies are negative. Renal ultrasound shows normal-sized kidneys. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: ANCA-Associated Vasculitis (RPGN) ### Clinical Presentation Analysis **Key Point:** Rapidly progressive glomerulonephritis (RPGN) is a nephrologic emergency characterized by rapid decline in renal function (days to weeks), hematuria with RBC casts, and crescent formation on biopsy. ANCA-associated vasculitis (AAV) is the most common cause in adults. ### Red Flags for RPGN in This Case 1. **Rapid renal deterioration** — creatinine 2.1 mg/dL in 1 week suggests acute kidney injury 2. **Nephritic presentation** — hematuria, RBC casts, hypertension, edema 3. **Significant proteinuria** — 2.5 g/day (nephritic-range) 4. **Pulmonary symptoms** — dyspnea suggests possible pulmonary involvement (pulmonary-renal syndrome) 5. **Negative serologies** — rules out anti-GBM and lupus ### Differential Diagnosis of RPGN | Feature | ANCA-AAV | Anti-GBM | Lupus | IgA + Crescents | |---------|----------|----------|-------|------------------| | **Serology** | ANCA+ (MPO or PR3) | Anti-GBM+ | ANA+, anti-dsDNA+ | Negative | | **C3/C4** | Normal | Normal | ↓ (especially C4) | Normal | | **Pulmonary involvement** | Common (hemoptysis, infiltrates) | Common (hemoptysis) | Variable | Rare | | **Systemic features** | Fever, arthralgia, rash | Rare | Rash, photosensitivity, arthritis | Rare | | **Renal biopsy** | Pauci-immune crescents | Linear IgG on GBM | Subendothelial deposits | IgA-dominant deposits | | **Prognosis without treatment** | ESRD in weeks | ESRD in days | Variable | Variable | **High-Yield:** The combination of **negative ANA, negative anti-GBM, normal complement, and rapid renal decline** is pathognomonic for pauci-immune RPGN (ANCA-associated vasculitis). ### Why ANCA-AAV is Most Likely 1. **Serology pattern:** Negative ANA and anti-GBM effectively exclude lupus and anti-GBM disease 2. **Normal complement:** Rules out immune complex-mediated RPGN (lupus, post-infectious) 3. **Rapid renal deterioration:** Creatinine doubled in 1 week — typical of RPGN 4. **Pulmonary symptoms:** Dyspnea suggests possible pulmonary-renal syndrome (common in AAV) 5. **Pauci-immune mechanism:** Normal complement + negative serologies = pauci-immune crescents **Clinical Pearl:** ANCA-associated vasculitis includes three entities: - **Granulomatosis with polyangiitis (GPA)** — upper respiratory + lung + kidney involvement - **Microscopic polyangiitis (MPA)** — lung + kidney, no upper respiratory involvement - **Eosinophilic granulomatosis with polyangiitis (EGPA)** — asthma, eosinophilia, systemic features ### Diagnostic Confirmation ```mermaid flowchart TD A[Suspected RPGN]:::outcome --> B[Check ANCA serology]:::action B --> C{ANCA positive?}:::decision C -->|Yes| D[Pauci-immune RPGN]:::outcome C -->|No| E[Perform kidney biopsy]:::action E --> F{Biopsy findings?}:::decision F -->|Pauci-immune crescents| G[ANCA-negative RPGN]:::outcome F -->|IgA deposits| H[IgA nephropathy with crescents]:::outcome F -->|Other pattern| I[Alternative diagnosis]:::outcome D --> J[Immunosuppression: corticosteroids + cyclophosphamide or rituximab]:::action J --> K[Plasmapheresis if pulmonary hemorrhage]:::action ``` **Mnemonic:** **RPGN causes = ANCA, Anti-GBM, Immune complex** - **ANCA** — pauci-immune (GPA, MPA, EGPA) - **Anti-GBM** — linear IgG on basement membrane - **Immune complex** — lupus, post-infectious, IgA ### Management Priority **Warning:** RPGN is a renal emergency. Every day of delay increases the risk of irreversible renal failure. Immunosuppression (corticosteroids + cyclophosphamide or rituximab) should be initiated immediately after kidney biopsy confirmation, not after waiting for ANCA results. **Key Point:** Plasmapheresis is indicated if there is evidence of pulmonary hemorrhage (hemoptysis, alveolar infiltrates on CXR, low Hgb) or if creatinine >5 mg/dL at presentation.