What is the role of ANCA detection in the workup of granulomatosis with polyangiitis (GPA)?

Updated: Oct 09, 2019
  • Author: Christopher L Tracy, MD; Chief Editor: Herbert S Diamond, MD  more...
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Answer

Antineutrophil cytoplasmic antibodies (ANCAs) can be detected with serologic assays. The 2 types of assays in common use are immunofluorescence (IF) and enzyme immunoassay.  A 2017 position paper by an international consensus panel stated that high-quality immunoassays are the preferred screening method for the diagnosis of ANCA-associated vasculitis, and that screening does not categorically require indirect IF assays. [53]

Three types of IF patterns are recognized: C-ANCA (cytoplasmic antibody), P-ANCA (perinuclear antibody), and atypical ANCA. (See the images below.)

C-ANCA immunofluorescence pattern. Staining for an C-ANCA immunofluorescence pattern. Staining for antineutrophil cytoplasmic antibody by indirect immunofluorescence shows heavy cytoplasmic staining, whereas nuclei are nonreactive. Image courtesy of K. Orr, MD.
P-ANCA immunofluorescence pattern. Perinuclear ant P-ANCA immunofluorescence pattern. Perinuclear antineutrophil cytoplasmic antibody staining pattern by indirect immunofluorescence shows perinuclear staining, whereas cytoplasm is nonreactive. Image courtesy of K. Orr, MD.

IF represents a qualitative ANCA assay, and significant inter-reader variability exists. Enzyme-linked immunosorbent assay (ELISA) provides target antigen-specific characterization of ANCA (ie, anti-PR3 and anti-myeloperoxidase [MPO]) and should be used to confirm IF findings. Combining IF and ELISA enhances the sensitivity and specificity of a diagnosis of AAV to 96% and 98.5%, respectively. [23] Only ANCAs directed against PR3 or MPO have been associated with primary vasculitic syndromes.

C-ANCA directed against PR3 is most specific for GPA. According to the WGET trial, IF shows positive C-ANCA results in 88% of all patients with GPA. IF shows positive C-ANCA results in 87% of patients with severe disease and in 90% of those with limited disease. [9] Using both IF and ELISA, ANCA is detectable in nearly 100% of patients with active generalized GPA. [23]

Some patients with GPA express P-ANCA specific for MPO. Analysis of the WGET cohort demonstrated that IF showed positive P-ANCA results in 13% of patients with severe disease; 10%of patients with limited disease were P-ANCA–positive. [9] A few patients with GPA are ANCA-negative, although this fraction appears to be exceedingly small. [10, 18]

Rising C-ANCA titers may herald a relapse in some patients with GPA, but this relationship is unreliable. Thus, patients with rising ANCA titers should not be treated with cytotoxic medications in the absence of signs, symptoms, or other objective evidence of disease relapse. [22, 37]

Further considerations in P-ANCA testing

In contrast to GPA, the indirect IF staining pattern in microscopic polyangiitis and allergic granulomatous angiitis is often perinuclear (P-ANCA). This is an artifactual phenomenon that occurs during the ethanol fixation process of neutrophils, resulting in the displacement of the basic positively charged proteins (eg, MPO, lactoferrin, lysozyme, elastase, cathepsin G) from the cytoplasm to the nuclear region. MPO is the antigen at which these autoantibodies are most often directed in the setting of small-vessel vasculitis. This antibody can be observed in microscopic polyangiitis, allergic granulomatous angiitis, idiopathic crescentic glomerulonephritis, and, occasionally, GPA.

A positive finding on a P-ANCA test alone can be observed in a number of other diseases that would not qualify as small-vessel vasculitis. These include inflammatory bowel disease, Kawasaki disease, polyarteritis nodosa, Felty syndrome, and infections such as human immunodeficiency virus (HIV) infection and endocarditis. Because of the variability of the P-ANCA target antigen, more specific antibody testing is recommended strongly. A positive P-ANCA test result should be used to diagnose small vessel vasculitis only when it is used in conjunction with a positive antimyeloperoxidase titer in the setting of high clinical suspicion.

Cautions

In summary, be cautious not to equate a positive ANCA test result with disease. When used appropriately, the ANCA test is a very powerful test with high degrees of sensitivity and specificity; however, when used in the wrong setting, it can lead to misdiagnosis with resultant inappropriate treatment using potentially toxic therapy. Indeed, a positive C-ANCA test result in patients with only sinusitis has a posttest probability of 7-16% of correctly diagnosing GPA. In patients with sinusitis, pulmonary infiltrates or nodules, and active urinary sediment with RBC casts, a positive C-ANCA test finding has a posttest probability of GPA of 98%. Moreover, ANCA tests should not be used to correlate with clinical disease in patients with established diagnoses of vasculitis.


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