A 58-year-old man presents with a 3-week history of fever, malaise, and progressive dyspnea. On examination, he has palpable purpura over the lower extremities and ankles, along with mild arthralgia of the knees. Urinalysis shows hematuria with RBC casts. Serum creatinine is 1.8 mg/dL. Which investigation is most appropriate to confirm the diagnosis of IgA vasculitis?

Explanation

## Diagnosis of IgA Vasculitis **Key Point:** Skin biopsy with immunofluorescence showing IgA-dominant deposition is the gold standard and most accessible confirmatory test for IgA vasculitis (formerly Henoch-Schönlein purpura). ### Clinical Presentation Recognition The patient presents with the classic tetrad of IgA vasculitis: - Palpable purpura (lower extremities, buttocks) - Arthralgia/arthritis (knees, ankles) - Renal involvement (hematuria, RBC casts, rising creatinine) - Systemic symptoms (fever, malaise) ### Why Skin Biopsy is the Investigation of Choice | Feature | Skin Biopsy | Renal Biopsy | ANCA Serology | |---------|------------|-------------|---------------| | **Accessibility** | Minimally invasive, bedside | Invasive, requires expertise | Non-invasive | | **Diagnostic yield** | >90% with IgA deposition | Gold standard for renal disease | Negative in IgA vasculitis | | **Timing** | Can be done immediately | Reserved for prognostic/therapeutic decisions | Not diagnostic | | **Risk** | Minimal | Bleeding, infection risk | N/A | **High-Yield:** IgA vasculitis is characterized by **IgA-dominant immune complex deposition** on immunofluorescence. The skin biopsy site should be from affected purpuric lesions or normal-appearing skin adjacent to purpura (both show IgA deposition). ### Immunofluorescence Pattern IgA vasculitis shows: - **Dominant IgA** in vessel walls and glomeruli - **Minimal or absent IgG and IgM** - This distinguishes it from other vasculitides (e.g., ANCA-associated vasculitis shows IgG/IgM, not IgA) **Clinical Pearl:** Renal biopsy, while definitive for assessing glomerulonephritis severity and prognosis, is NOT the initial confirmatory test—it is reserved for patients with significant renal involvement who may require immunosuppressive therapy. **Mnemonic:** **IgA-VAS** = IgA Vasculitis—Assess with **Skin** biopsy first (Immunofluorescence confirms IgA-dominant deposition). [cite:Harrison 21e Ch 319]