Concurrent Anti-PR3 Immunoassay and cANCA Indirect Immunofluorescence Testing Provide Complementary Information for...

Multiplex immunoassays and indirect immunofluorescence assays (IFA) are frequently used for the detection of antineutrophil cytoplasmic antibodies (ANCA) but can produce differing results.

To understand the settings in which discordant results occur, we retrospectively investigated first-recorded cytoplasmic ANCA (cANCA) IFA and antiproteinase 3 (PR3) results (n = 15 772) generated over 4 years in our clinical laboratory.

We detected significant bidirectional disagreement between cANCA and anti-PR3-antibody results that was not immunoassay or instrument specific. Sixteen percent (22/139) of cANCA-positive specimens were negative for anti-PR3 by multiplex immunoassay. Similarly, 61% (175/285) of anti-PR3 positive specimens were negative for cANCA by IFA. Borderline or indeterminate positivity thresholds could not fully explain the discrepancy between results, suggesting that patient-associated characteristics may contribute. Medical record review of patients with discordant testing over time revealed cANCA/anti-PR3 disagreement in patients with ANCA-associated vasculitis on chronic immunomodulatory therapy, suggesting a treatment-related phenomenon. Among 59% (13/22) of patients for whom serial ANCA testing was evaluated, cANCA semiquantitative titers and anti-PR3 antibody quantitative results trended together and correlated with disease activity in response to immunomodulatory therapy. However, asynchronous changes were observed in 41% (9/22) of patients, leading to significant differences in anti-PR3 or cANCA results despite sustained cANCA or anti-PR3 antibody negativity, respectively.

In certain individuals, anti-PR3 antibodies and cANCA titers may differentially capture disease activity, and cANCA/anti-PR3-discordant results may have clinical significance in the setting of immunomodulatory therapy. Our data argue for a complementary testing approach that implements both immunoassay and IFA methods to detect these changes over time.