A 58-year-old man presents with a 3-week history of fever, malaise, and progressive dyspnea. Examination reveals palpable purpura over the lower extremities and ankles, along with mild arthralgia of the knees. Chest X-ray shows bilateral lower lobe infiltrates. Urinalysis reveals 2+ proteinuria and RBC casts. Which investigation is most specific for confirming the diagnosis of IgA vasculitis (IgAV)?

Explanation

## Diagnosis of IgA Vasculitis ### Gold Standard Investigation **Key Point:** Skin biopsy with immunofluorescence (IF) microscopy showing predominant IgA deposition is the gold standard and most specific diagnostic test for IgA vasculitis (formerly Henoch–Schönlein purpura). ### Why Immunofluorescence is Superior **High-Yield:** IgA vasculitis is characterized by IgA-dominant immune complex deposition in small vessels. Immunofluorescence microscopy directly visualizes this IgA deposition in: - Skin vessels (even in non-purpuric areas) - Glomeruli (on renal biopsy) - Mesangium of kidneys ### Diagnostic Criteria & Sensitivity | Investigation | Sensitivity | Specificity | Notes | | --- | --- | --- | --- | | Skin biopsy + IF | 95–100% | Highly specific | Gold standard; can use non-purpuric skin | | Serum IgA level | 50–60% | Low | Elevated IgA is neither sensitive nor specific | | ANCA panel | Negative | N/A | ANCA-negative in IgAV; rules out ANCA-associated vasculitis | | Renal biopsy (light only) | Variable | Low | Light microscopy shows proliferative GN but cannot confirm IgA | ### Clinical Pearl **Clinical Pearl:** Skin biopsy can be taken from purpuric OR non-purpuric areas (e.g., buttocks, thigh) and is less invasive than renal biopsy, making it the preferred confirmatory test in clinical practice. ### Diagnostic Algorithm ```mermaid flowchart TD A[Clinical suspicion: palpable purpura + arthralgia + GN]:::outcome A --> B[Perform skin biopsy with IF]:::action B --> C{IgA deposition on IF?}:::decision C -->|Yes| D[IgA vasculitis confirmed]:::outcome C -->|No| E[Consider alternative diagnosis]:::outcome A --> F[Serum IgA level]:::action F --> G[Elevated but non-specific]:::outcome ``` ### Why Other Options Are Incorrect **Serum IgA level:** While elevated in ~50% of IgAV patients, it is neither sensitive nor specific and cannot confirm diagnosis. **ANCA panel:** Typically negative in IgAV; used to exclude ANCA-associated vasculitis (GPA, MPA). **Renal biopsy (light microscopy alone):** Shows IgA nephropathy (proliferative GN) but requires immunofluorescence to confirm IgA deposition; more invasive than skin biopsy.