ANCA are autoantibodies directed against antigens found in cytoplasmic granules of neutrophils and monocytes. ANCA are classified according to the immunfluorescence patterns they produce on normal neutrophils and according to their target antigens. At least two fluorescence patterns can be differentiated: a granular fluorescence which is distributed regularly over the entire cytoplasm of the granulocytes, leaving the cell nuclei free (cANCA: cytoplasmic pattern), and a predominantly smooth, partly fine granular fluorescence wrapped ribbon-like around the cell nuclei of the granulocytes (pANCA: perinuclear pattern). ANCA are typically found in granulomatosis with polyangiitis (Wegener’s) (GPA), microscopic polyangiitis (MPA) including renal limited vasculitis, and Churg-Strauss syndrome (CSS), which are all forms of small-vessel vasculitis. These three diseases are grouped together as ANCA-associated vasculitides (AAV). The most important clinical symptoms of ANCA-associated vasculitides are caused by poor blood supply to organs or formation of aneurysms and bleeding due to destruction of blood vessels. Classical cANCA are present in most patients with GPA and in about 30% of patients with MPA. Classical cANCA is almost always directed against proteinase 3 (PR3), and very rarely against myeloperoxidase (MPO) or against both PR3 and MPO simultaneously. In patients with MPA and CSS, myeloperoxidase (MPO) is the main target antigen of pANCA.

Current criteria for the diagnosis of GPA, MPA and CSS rely on histology and do not take ANCA status or antibody specificity into account. However, histology is not always conclusive, and clinicians are using ANCA to help confirm or exclude the diagnosis of small-vessel vasculitis, and to monitor inflammatory activity in these diseases. The demonstration of ANCA antibodies is helpful in both the diagnosis and management of GPA and MPA. ANCA levels are usually high at presentation, fall with treatment and often increase prior to clinical relapse. A rather good association of ANCA titers with disease activity has been described by many scientific researchers. However, increasing ANCA titers do not reliably predict relapse. The role of ANCA in monitoring disease activity in patients with vasculitis and the significance of changes in ANCA titers for treatment remains unclear