Info from Brighton & Sussex Pathology:
In general in the context of small vessel vasculitis:
C-ANCA is associated with anti-PR3 antibodies and is found in Wegener’s Granulomatosis, and Churg-Strauss Syndrome.
P-ANCA is associated with anti-MPO antibodies and is found in Microscopic Polyangiitis, Crescentic Glomerulonephritis and Churg-Strauss Syndrome.
But this is variable and one may have C-ANCA with anti-MPO and P-ANCA with anti-PR3, or combinations of antibodies.
A positive ANCA with negative anti-MPO and PR3 may be found in a variety of conditions including autoimmune hepatitis, sclerosing cholangitis, ulcerative colitis, SLE, RA, malignancy and chronic infections. ANCA testing is not warranted for these clinical conditions.
My doctor wanted to start me on methotrexate but I don't like the thought of starting that drug without some sort of definitive diagnosis.