A 35-year-old woman with a known history of systemic lupus erythematosus (SLE) presents with gross haematuria, proteinuria (2.5 g/day), and hypertension for 2 weeks. Serum creatinine is 1.8 mg/dL. Anti-dsDNA antibodies are strongly positive, and complement C3 and C4 levels are low. Which investigation is most appropriate to determine the class of lupus nephritis and guide treatment decisions?

Explanation

## Renal Biopsy — Gold Standard for Lupus Nephritis Classification **Key Point:** Renal biopsy is the ONLY investigation that definitively establishes the class of lupus nephritis (I–VI), which directly determines prognosis and treatment intensity. ### WHO/ISN-RPS Classification of Lupus Nephritis | Class | Pathology | Light Microscopy | IF Pattern | Clinical Significance | |-------|-----------|------------------|-----------|----------------------| | **I** | Minimal | Normal or mesangial hypercellularity | Mesangial IgG, IgM, C3 | Excellent prognosis | | **II** | Mesangial proliferative | Mesangial proliferation | Mesangial deposits | Good prognosis | | **III** | Focal proliferative | <50% glomeruli involved, endocapillary/extracapillary proliferation | Full house pattern (IgG, IgM, IgA, C3, C1q) | Requires immunosuppression | | **IV** | Diffuse proliferative | ≥50% glomeruli involved, wire-loop lesions, hyaline thrombi | **Full house** (most common) | Most severe; high risk of ESRD | | **V** | Membranous | Capillary wall thickening, subepithelial deposits (spike-and-dome) | Subepithelial IgG, C3 | Risk of nephrotic syndrome | | **VI** | Advanced sclerosis | Sclerosed glomeruli (>90%) | Minimal/absent deposits | ESRD; limited treatment response | **High-Yield:** The **"full house" immunofluorescence pattern** (simultaneous IgG, IgM, IgA, C3, C1q deposits) is virtually pathognomonic for lupus nephritis and distinguishes it from other causes of nephritic syndrome. ### Why Biopsy Is Essential in This Case 1. **Class determines therapy intensity:** - Class I–II: Conservative management (NSAIDs, antimalarials) - Class III–IV: Aggressive immunosuppression (cyclophosphamide, mycophenolate mofetil, corticosteroids) - Class V: ACE inhibitors/ARBs + immunosuppression - Class VI: Supportive care only 2. **Prognosis varies by class:** - Class IV (diffuse proliferative) has worst prognosis (40–50% progress to ESRD without treatment) - Class I–II have excellent outcomes 3. **Guides monitoring:** Repeat biopsy may be needed to assess treatment response or detect class transition **Clinical Pearl:** Anti-dsDNA and low C3/C4 are markers of disease activity but do NOT determine nephritis class. A patient can have Class I nephritis with positive anti-dsDNA and low complements, or Class IV with normal serology — only biopsy reveals the truth. **Mnemonic: FULL HOUSE** — **F**ibrinogen, **U**nidentified (C1q), **L**ow complement (C3), **L**arge IgG, **H**igh IgM, **O**ther Igs (IgA), **U**niversal C3, **S**imultaneous deposits, **E**verywhere (all compartments)