A 58-year-old man with a 6-month history of nephrotic syndrome (proteinuria 5.2 g/day, serum albumin 2.3 g/dL) and progressive renal insufficiency (serum creatinine 2.8 mg/dL, eGFR 22 mL/min) undergoes renal biopsy. Light microscopy shows thickened glomerular basement membrane with a 'spike and dome' appearance on electron microscopy. Immunofluorescence shows granular IgG and C3 deposits along the capillary wall. Which additional investigation is most appropriate to identify the underlying etiology?

Explanation

## Membranous Nephropathy (MN) — Etiology Workup ### Histopathological Diagnosis The biopsy findings are **pathognomonic for membranous nephropathy (MN)**: | Feature | Finding | Significance | | --- | --- | --- | | **Light Microscopy** | Thickened GBM, no proliferation | Characteristic of MN | | **Electron Microscopy** | "Spike and dome" appearance | Subepithelial electron-dense deposits with GBM projections — hallmark of MN | | **Immunofluorescence** | Granular IgG + C3 on capillary wall | Immune complex deposition along GBM | **Key Point:** Once MN is confirmed histologically, the next step is to identify the **etiology** — primary (idiopathic) vs. secondary (associated with systemic disease, infection, malignancy, or drugs). ### Anti-PLA2R Antibody — Why This Test? **High-Yield:** Anti-phospholipase A2 receptor (PLA2R) antibodies are found in **70–80% of patients with primary (idiopathic) MN** and are rarely present in secondary MN. **Clinical Pearl:** - **Positive anti-PLA2R** → strongly suggests **primary MN** (idiopathic) - **Negative anti-PLA2R** → raises suspicion for **secondary MN** (infection, malignancy, SLE, drugs) - Anti-PLA2R status also predicts prognosis: positive patients have lower remission rates with immunosuppression ### Diagnostic Algorithm for MN Etiology ```mermaid flowchart TD A["Renal biopsy confirms MN<br/>(spike and dome, granular IgG+C3)"]:::outcome --> B["Anti-PLA2R antibody testing"]:::action B --> C{"Anti-PLA2R positive?"}:::decision C -->|Yes| D["Primary MN<br/>(idiopathic)"]:::outcome C -->|No| E["Likely secondary MN<br/>Screen for etiology"]:::outcome E --> F["Hepatitis B/C serology"]:::action E --> G["Malignancy screening<br/>(age >50)"]:::action E --> H["SLE serology<br/>(ANA, anti-dsDNA)"]:::action E --> I["Drug history<br/>(NSAIDs, gold, penicillamine)"]:::action ``` **Mnemonic: "PLA2R First" for MN workup** — Anti-**P**hospholipase **A2** **R**eceptor is the first-line test to distinguish primary from secondary MN. ### Why Anti-PLA2R Over Other Tests? Anti-PLA2R is the **most specific and sensitive serological marker** for primary MN and should be checked **first** in all patients with biopsy-proven MN. It is more specific than general screening tests (ANA, hepatitis serology) which are checked only if anti-PLA2R is negative or if clinical features suggest secondary disease.