USMLE Step 1 · Immunology

c-ANCA pattern vs p-ANCA pattern

How to tell c-ANCA pattern and p-ANCA pattern apart when a board question makes them look alike.

Fastest separator

c-ANCA/PR3: granulomatosis with polyangiitis (GPA); p-ANCA/MPO: microscopic polyangiitis and eosinophilic granulomatosis with polyangiitis.

Side by side

c-ANCA patternp-ANCA pattern

Key clue

c-ANCA usually targets PR3 and classically supports granulomatosis with polyangiitis with ENT, lung, and kidney granulomatous vasculitis.

p-ANCA usually targets MPO and supports microscopic polyangiitis, eosinophilic granulomatosis with polyangiitis, ulcerative colitis, or primary sclerosing cholangitis contexts.

Mechanism

C-ANCA usually targets PR3 and classically supports granulomatosis with polyangiitis with ENT, lung, and kidney granulomatous vasculitis.

P-ANCA usually targets MPO and supports microscopic polyangiitis, eosinophilic granulomatosis with polyangiitis, ulcerative colitis, or primary sclerosing cholangitis contexts.

Labs and tests

Anti-PR3 on ELISA, cytoplasmic staining on indirect immunofluorescence, saddle nose deformity, cavitary lung nodules, and pauci-immune crescentic glomerulonephritis support c-ANCA disease.

Anti-MPO on ELISA, perinuclear staining on indirect immunofluorescence, no granulomas in microscopic polyangiitis, and asthma with eosinophilia in EGPA support p-ANCA disease.

The trap

c-ANCA/PR3 points GPA; p-ANCA/MPO points MPA/EGPA and some GI autoimmune associations.

The repair move

Before naming the answer, force the stem through the separator: c-ANCA/PR3 points GPA; p-ANCA/MPO points MPA/EGPA and some GI autoimmune associations.

Try a free question

Core

A 48-year-old man has 6 months of chronic sinusitis with bloody nasal discharge, saddle nose deformity, hemoptysis with CT showing cavitary lung lesions, and acute kidney injury with active urinary sediment. Kidney biopsy shows pauci-immune crescentic glomerulonephritis. Indirect immunofluorescence shows a cytoplasmic ANCA pattern; ELISA confirms anti-proteinase 3 (PR3). Which condition is most likely?

Show answer and explanation

Answer: A. Granulomatosis with polyangiitis (GPA, formerly Wegener). Granulomatosis with polyangiitis (GPA) features c-ANCA targeting proteinase 3 (PR3). Triad: upper airway (sinusitis, saddle nose, otitis), lower airway (cavitary lung lesions, hemoptysis), and kidney (pauci-immune crescentic GN). Treat with rituximab plus corticosteroids.

Why the other choices are tempting

B. Microscopic polyangiitis has p-ANCA targeting MPO with pulmonary capillaritis and pauci-immune GN but NO upper airway granulomas.
C. EGPA has p-ANCA in about half of patients plus asthma, eosinophilia, and mononeuritis multiplex.
D. Polyarteritis nodosa is medium-vessel vasculitis with no ANCA association; HBV-associated.
E. Goodpasture syndrome has anti-GBM antibody causing pulmonary hemorrhage and rapidly progressive GN with linear IgG deposition, not pauci-immune.

Flashcards

Tap a card to flip it.

What clue supports c-ANCA pattern?

Choose c-ANCA pattern when the stem gives this pattern: c-ANCA usually targets PR3 and classically supports granulomatosis with polyangiitis with ENT, lung, and kidney granulomatous vasculitis.

What clue supports p-ANCA pattern?

Choose p-ANCA pattern when the stem gives this contrasting pattern: p-ANCA usually targets MPO and supports microscopic polyangiitis, eosinophilic granulomatosis with polyangiitis, ulcerative colitis, or primary sclerosing cholangitis contexts.

Quick answers

What is the fastest way to tell c-ANCA pattern from p-ANCA pattern?