## ANCA-Associated Vasculitides: Serological Classification **Key Point:** Microscopic polyangiitis (MPA) is characterized by: - **p-ANCA positivity** (perinuclear pattern) - **Anti-MPO antibodies** (myeloperoxidase) - Small-vessel necrotizing vasculitis - Absence of granulomas ### ANCA Pattern and Antigen Correlation | Vasculitis | ANCA Pattern | Target Antigen | Vessel Size | Granulomas | |------------|--------------|----------------|-------------|------------| | **MPA** | **p-ANCA** | **MPO** | Small | No | | **GPA** | **c-ANCA** | **PR3** | Small | Yes | | **EGPA** | p-ANCA (variable) | MPO or PR3 | Small | Yes | | **PAN** | Negative | — | Medium | Yes | **High-Yield:** The mnemonic **"MPO = p-ANCA, PR3 = c-ANCA"** is the most testable fact in ANCA-associated vasculitis. ### Immunological Basis **Mnemonic:** **MPO-p-ANCA** - **M**yeloperoxidase → **p**erinuclear (p-ANCA) - **PR3** (Proteinase-3) → **c**ytoplasmic (c-ANCA) ### Clinical Distinction: MPA vs GPA | Feature | MPA | GPA | |---------|-----|-----| | ANCA | p-ANCA/MPO | c-ANCA/PR3 | | Granulomas | **Absent** | Present | | Respiratory involvement | Pulmonary hemorrhage only | Upper + lower respiratory tract | | Glomerulonephritis | Pauci-immune RPGN | Pauci-immune RPGN | | Extravascular granulomas | No | Yes (necrotizing granulomas) | **Clinical Pearl:** MPA is the most common ANCA-associated vasculitis, accounting for ~50% of cases. It typically presents with: - Rapidly progressive glomerulonephritis (RPGN) - Pulmonary hemorrhage - Systemic symptoms (fever, arthralgia, myalgia) ### Why This Matters **Warning:** Do NOT confuse p-ANCA with p-ANCA/MPO. Some p-ANCA patterns are associated with other antigens (e.g., elastase, lactoferrin), but in the context of vasculitis, **p-ANCA = MPO** until proven otherwise. [cite:Robbins 10e Ch 11; Harrison 21e Ch 319] 
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