A 35-year-old woman with a 3-week history of haematuria, proteinuria (3.5 g/day), and acute kidney injury (serum creatinine 2.8 mg/dL) presents with dyspnoea and chest pain. Chest X-ray shows pulmonary infiltrates. ANCA serology is positive for c-ANCA (anti-PR3). Which investigation is most appropriate to confirm the diagnosis of ANCA-associated vasculitis and guide treatment?

Explanation

## Diagnostic Confirmation in ANCA-Associated Vasculitis (AAV) ### Clinical Presentation The patient presents with: - Rapidly progressive glomerulonephritis (RPGN): acute rise in creatinine, haematuria, proteinuria - Pulmonary involvement: dyspnoea, chest pain, infiltrates on CXR (suggestive of pulmonary haemorrhage or granulomatosis) - Positive c-ANCA/anti-PR3 serology (highly specific for granulomatosis with polyangiitis, GPA) This constellation suggests **ANCA-associated vasculitis with pulmonary-renal involvement**. ### Investigation Hierarchy in Suspected AAV | Investigation | Purpose | Role in AAV | |---|---|---| | **Kidney biopsy** | Histological confirmation; demonstrates pauci-immune crescentic GN | **Gold standard** for diagnosis; shows necrotizing GN with minimal/absent immune deposits | | ANCA serology (c-ANCA/anti-PR3) | Serological marker | Highly specific but not diagnostic alone; can be negative in 10–15% of AAV | | Anti-GBM antibody | Diagnostic for anti-GBM disease | Rules out anti-GBM disease (linear IgG on IF); negative in AAV | | HRCT chest | Structural lung imaging | Identifies pulmonary granulomas or infiltrates; supports diagnosis but non-specific | | Cryoglobulin level | Diagnostic for cryoglobulinaemia | Irrelevant in AAV; seen in mixed cryoglobulinaemia (HCV-related) | ### Why Kidney Biopsy Is Definitive **Key Point:** Kidney biopsy is the gold standard for confirming ANCA-associated vasculitis and demonstrating the pathognomonic pauci-immune crescentic glomerulonephritis. **Light microscopy findings in AAV:** - Segmental fibrinoid necrosis of glomeruli - Crescent formation (cellular, fibrocellular, or fibrous) - Minimal endocapillary proliferation - Tubular atrophy and interstitial fibrosis (in chronic disease) **Immunofluorescence findings (pathognomonic):** - **Pauci-immune pattern**: minimal or absent immunoglobulin and complement deposits despite severe necrosis - This absence of immune deposits distinguishes AAV from immune complex GN (PSGN, lupus, IgA nephropathy) **Electron microscopy findings:** - Confirms absence of electron-dense immune deposits - Shows fibrinoid necrosis and crescent formation **Clinical Pearl:** The hallmark of AAV is the **pauci-immune crescentic GN**—severe necrotizing glomerulonephritis with minimal immune deposits on IF. This pattern, combined with positive ANCA serology and clinical features (pulmonary involvement, systemic vasculitis), confirms AAV. **High-Yield:** While c-ANCA/anti-PR3 serology is highly specific for GPA, it can be negative in 10–15% of patients with biopsy-proven AAV. Therefore, kidney biopsy remains the diagnostic gold standard, especially in RPGN with pulmonary involvement. **Mnemonic: ANCA-AAV Biopsy Pattern — "PAUCI"** - **P**auci-immune (minimal Ig/C deposits) - **A**cute fibrinoid necrosis - **U**sually crescents present - **C**rescent formation (cellular/fibrocellular) - **I**mmunofluorescence negative [cite:Robbins 10e Ch 20]