The diagnosis of GPA is established by clinical and laboratory findings such as the ANCA blood test, other blood and urine tests, x-rays and tissue biopsy, if needed. Antineutrophil Cytoplasmic Antibody (ANCA) is an abnormal protein. ANCA is part of a large family of molecules called immunoglobulins, (including antibodies), that are made by all animals and are normally intended to protect you. There are two types of ANCA: ‘c’ (cytoplasmic), and ‘p’ (perinuclear). The great majority of patients with GPA test positive for c-ANCA while a small percentage of patients test positive for both ‘p’-and ‘c’- ANCA. C-ANCA reacts with a normal human enzyme contained in white blood cells (called proteinase 3 or PR3), and has a yellow-green pattern in the cell fluid called the cytoplasm, hence the term c-ANCA. The quantity of c-ANCA roughly correlates with disease activity. Very rarely, C-ANCA can be found in other diseases and even in some normal individuals. The c-ANCA/PR3 antibody test is a helpful diagnostic tool that is used most effectively when patients are thought to possibly have GPA. A positive test is supportive of the diagnosis. However, occasionally the test is negative but GPA is present.
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