p-ANCA

p-ANCA (Perinuclear Anti-Neutrophil Cytoplasmic Antibody)

p-ANCA refers to a pattern of staining observed in indirect immunofluorescence (IIF) assays used to detect anti-neutrophil cytoplasmic antibodies (ANCAs). ANCAs are autoantibodies directed against proteins within the cytoplasm of neutrophils and, to a lesser extent, monocytes. The "p" prefix indicates a perinuclear staining pattern, meaning the antibody binding is concentrated around the nucleus of the neutrophil. This is an artifact of the ethanol fixation process used in the IIF assay, causing the redistribution of certain target antigens from the cytoplasm to the perinuclear region.

The p-ANCA pattern, while visually distinctive, is not specific for a single disease or target antigen. It is often associated with antibodies against myeloperoxidase (MPO), an enzyme found in the azurophilic granules of neutrophils. However, other antigens can also give rise to a p-ANCA pattern, including lactoferrin, elastase, cathepsin G, and bactericidal/permeability-increasing protein (BPI). In some cases, the antigen target may not be identifiable.

The clinical significance of p-ANCA positivity depends on the specific antibody target and the clinical context. p-ANCA, particularly when associated with anti-MPO antibodies, is strongly linked to certain autoimmune vasculitides, especially microscopic polyangiitis (MPA) and eosinophilic granulomatosis with polyangiitis (EGPA, formerly known as Churg-Strauss syndrome). It can also be found in other conditions such as ulcerative colitis, primary sclerosing cholangitis, and drug-induced vasculitis.

Due to the potential for multiple target antigens producing the same pattern, it is crucial to confirm the specificity of a p-ANCA result using antigen-specific assays such as ELISA (enzyme-linked immunosorbent assay) or multiplex assays that directly measure antibodies against specific target antigens. These assays are necessary to determine the clinical relevance of a positive p-ANCA result and guide appropriate diagnosis and management. A negative antigen-specific assay despite a p-ANCA pattern suggests the antibody targets are less common or unidentified.