Clinical Information

Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitides are characterized by a pauci-immune inflammation within the walls of small blood vessels (1). There are 3 specific diseases which are identified as ANCA-associated vasculitides: microscopic polyangiitis (MPA), granulomatosus with polyangiitis (GPA), and eosinophilic granulomatosus with polyangiitis (EGPA). The serological hallmark of these disorders is the presence of ANCA, which are antibodies that bind to cytoplasmic antigens found in the granules of neutrophils (2). Patients with GPA frequently have antibodies specific for proteinase 3 (PR3), while individuals with MPA or EGPA are more likely to have antibodies that bind to myeloperoxidase (MPO). The presence of PR3-ANCA and MPO-ANCA can be detected using antigen-specific immunoassays or indirect immunofluorescence (IIF). IIF is performed most commonly using ethanol-fixed neutrophils. Using this substrate, anti-PR3 antibodies produce a granular cytoplasmic staining pattern, which is referred to as cANCA. In comparison, due to an artefact that is a result of the fixation process, anti-MPO antibodies display a perinuclear pattern, or pANCA.

Patients with suspected ANCA-associated vasculitis should be evaluated for the presence of PR3-ANCA, MPO-ANCA and ANCA by IIF. A consensus guideline published in 2017 recommends that patients with possible GPA or MPA be tested for PR3-ANCA and MPO-ANCA using antigen-specific immunoassays (3). ANCA by IIF should then be used in cases where there is a high degree of suspicion for GPA or MPA but the PR3-ANCA and MPO-ANCA testing is negative. This guideline also suggests that ANCA may be used in situations where a low positive PR3-ANCA or MPO-ANCA is detected, to improve specificity of the testing. The classification criteria for MPA, GPA and EGPA published by the American College of Rheumatology and the European Alliance of Associations for Rheumatology include PR3-ANCA and MPO-ANCA detected by either antigen-specific immunoassay or IIF (4-6). These classification criteria incorporate serological ANCA testing along with clinical symptoms, imaging, and biopsy results to determine a score which allows for the classification of the various ANCA-associated vasculitides.