What ANCA Tests Detect
ANCA (antineutrophil cytoplasmic antibody) tests detect autoantibodies directed against proteins in neutrophil and monocyte granules—specifically proteinase 3 (PR3) and myeloperoxidase (MPO)—and are used primarily to diagnose ANCA-associated vasculitis (AAV), which includes granulomatosis with polyangiitis (GPA), microscopic polyangiitis (MPA), and eosinophilic granulomatosis with polyangiitis (EGPA). 1
Primary Clinical Purpose
ANCA testing serves as the key diagnostic biomarker for small-vessel vasculitis. About 90% of patients with active GPA and MPA test positive for ANCA, though this drops to approximately 30-40% in EGPA 2, 1. The test identifies two main antibody types:
- PR3-ANCA: Found in 84-85% of GPA patients and 2-27% of MPA patients
- MPO-ANCA: Found in 16% of GPA patients and 75-97% of MPA patients 2
Testing Methodology
High-quality antigen-specific immunoassays for PR3 and MPO are the recommended primary screening method 2, 1. The 2017 international consensus prioritized these direct immunoassays over traditional indirect immunofluorescence (IIF) patterns (C-ANCA and P-ANCA) due to superior diagnostic accuracy 2.
If immunoassay is negative but clinical suspicion remains high, a second test (another immunoassay or IIF) should be performed 2.
Clinical Scenarios for ANCA Testing
Test for ANCA when evaluating:
- Pulmonary-renal syndrome (simultaneous lung and kidney injury)—this presentation demands urgent evaluation as it may indicate rapidly progressive disease 1
- Rapidly declining kidney function with active urinary sediment (dysmorphic RBCs, red cell casts)
- Unexplained multi-system inflammatory disease affecting upper/lower respiratory tract, kidneys, skin, eyes, or nervous system
- Pauci-immune necrotizing crescentic glomerulonephritis on biopsy 1
Critical Diagnostic Caveats
ANCA positivity alone does not establish the diagnosis of AAV—clinical context is essential, as ANCA can appear in other inflammatory diseases, infections, and drug-induced conditions 2. Conversely, ANCA negativity does not exclude AAV: 10-20% of GPA/MPA patients and 40-50% of EGPA patients are ANCA-negative 2.
In pulmonary-renal syndrome, also test for anti-GBM antibodies, as dual positivity for ANCA and anti-GBM indicates overlap syndrome with worse renal prognosis 2, 1.
Treatment Implications
When clinical presentation is compatible with small-vessel vasculitis and MPO- or PR3-ANCA is positive, do not delay immunosuppressive therapy while waiting for biopsy confirmation, especially in rapidly deteriorating patients 1. The combination of positive ANCA serology with compatible clinical features has sufficient diagnostic specificity to initiate treatment immediately.
Disease Subclassification and Prognosis
ANCA serotype (PR3 vs. MPO) provides more prognostic information than clinical diagnosis alone. PR3-ANCA patients have higher relapse rates and different genetic backgrounds compared to MPO-ANCA patients, which may influence long-term treatment decisions 2.