ANCA are antibodies produced by the body against polynuclear neutrophils, a class of the body’s own white blood cells. The two main targets of these antibodies are PR3 (proteinase 3) and MPO (myeloperoxydase), proteins found in the cytoplasm (part of the cell outside the nucleus) of neutrophils. Under a microscope, p-ANCAs are grouped around the nucleus (perinuclear) while c-ANCAs are farther away (cytoplasmic). PR3 and MPO c-ANCA test results are presented as a numerical index and an interpretation (positive, negative or inconclusive). ANCA tests are useful in diagnosing and monitoring vasculitis, inflammation of the blood vessels, veins and arteries. Several other tests accompany or precede ANCA screening, such as blood count, sedimentation rate and other inflammation markers, verification of renal function with creatinine, etc.
The types of ANCA (perinuclear or cytoplasmic), their target (anti-PR3 or anti-MPO) and their relative levels (generally high index) vary according to the autoimmune disease involved (Wegener’s granulomatosis, microscopic polyangiitis, Churg & Strauss’s syndrome, polyarteritis nodosa, etc.). To clarify the diagnosis, a biopsy of a lesion is often necessary. The absence of these antibodies in the blood makes a diagnosis of autoimmune vasculitis less likely.