Laboratory investigations - antineutrophil cytoplasmic antibodies (ANCA)

Definition

ANCA - autoantibodies against enzymes in neutrophil cytoplasmic granules - a key serological marker in small-vessel vasculitis, distinct from the immune-complex-mediated vasculitides

Two main immunofluorescence patterns with corresponding target antigens; specific antigen testing (ELISA) is now preferred over pattern alone for clinical interpretation
c-ANCA (cytoplasmic pattern) - typically targets proteinase 3 (PR3) - associated with granulomatosis with polyangiitis (GPA)
p-ANCA (perinuclear pattern) - typically targets myeloperoxidase (MPO) - associated with microscopic polyangiitis and eosinophilic granulomatosis with polyangiitis (EGPA)
ANCA is thought to directly activate primed neutrophils, causing degranulation and endothelial injury - not simply a bystander marker

Report both the immunofluorescence pattern AND the specific antigen (PR3 vs MPO) - antigen specificity correlates better with clinical phenotype and relapse risk than IF pattern alone
A positive ANCA supports but does not alone confirm vasculitis - tissue biopsy remains important, especially when the diagnosis will commit the patient to long-term immunosuppression

Titre is used to support diagnosis at presentation; serial titre is not a reliable guide to disease activity/relapse on its own and should not solely drive treatment changes

Granulomatosis with polyangiitis, microscopic polyangiitis, EGPA, pulmonary-renal syndrome (with alveolar haemorrhage + glomerulonephritis)

A small proportion of clinically and histologically confirmed ANCA-associated vasculitis is ANCA-negative - a negative result does not exclude the diagnosis in a strongly suggestive clinical picture

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