ANCA testing has become the standard laboratory test for detecting and quantifying antibodies involved in microscopic angiitis. The three conditions usually sought when testing for ANCA are Wegener’s granulomatosis, Churg-Strauss syndrome and microscopic polyangiitis.

Two international Consensus Statements currently form the basis for optimizing the detection and characterization of these important autoantibodies 1, 2. There are two main patterns of ANCA reactivity that are found on substrates of peripheral blood neutrophils that have been fixed in ethanol: cytoplasmic-(C) ANCA and perinuclear-(P) ANCA. The C-ANCA pattern is due to reactivity with serine protease 3 (PR3) and is usually found in patients with Wegener’s granulomatosis. The P-ANCA pattern is due to reactivity with myeloperoxidase and is most often found in microscopic polyangiitis and Churg-Strauss syndrome. The consensus conference recommends that ANCA reactivity on ethanolfixed slides be confirmed by enzyme immunoassay (EIA) to confirm ANCA specific for proteinase 3 (PR3) and another to confirm ANCA specific for myeloperoxidase (MPO).

When these tests are used on serum from patients with symptoms of microscopic angiitis and show characteristic patterns of cytoplasmic (C) ANCA or perinuclear (P) ANCA that are confirmed by EIA to have PR3 and MPO reactivity, respectively, they are highly specific. However, false positive staining patterns can be seen in infectious and autoimmune conditions 3.

False positives are more likely when unusual patterns of reactivity are seen. For instance, if a C-ANCA pattern on ethanol fixed neutrophils is negative for PR3, but positive for P-ANCA, this is more likely to be a false positive than if it had been confirmed by MPO. Such false positive patterns have been reported in cases of infection by human immunodeficiency virus (HIV) 4

To minimize false positive reactivities, McLaren et al recommend requiring that appropriate symptoms such as pulmonary-renal syndrome, vasculitis of the skin with systemic complaints, chronic destructive airway disease, pulmonary nodules, subglottic stenosis of the trachea and mononeuritis multiplex be present 5.

When a discrepant confirmation is present, such as a positive MPO with a C-ANCA pattern, while it may be a case of Wegener’s granulomatosis, I recommend extra scrutiny to exclude other possibilities, especially infectious conditions such as HIV.