Antineutrophil cytoplasmic antibodies (ANCA) are autoantibodies produced by the immune system that mistakenly target and attack specific proteins within neutrophils (a type of white blood cell). ANCA testing detects and measures the amount of these autoantibodies in the blood. Two of the most common ANCAs are the autoantibodies that target the proteins myeloperoxidase (MPO) and proteinase 3 (PR3). These are called pANCAs. Tests for antineutrophil cytoplasmic antibodies (ANCA) may be used to: Help detect and diagnose certain forms of autoimmune vasculitis, including granulomatosis with polyangiitis, Help distinguish between ulcerative colitis (UC) and Crohn disease (CD), two common types of inflammatory bowel disease (IBD).