Diagnostic Performance of Myeloperoxidase and Proteinase-3 Antibodies by ImmunoCAP Fluorescent Enzyme Immunoassay

Introduction Anti-neutrophil cytoplasmic autoantibody (ANCA)-associated vasculitides (AAV) are traditionally diagnosed by positive ANCA on indirect immunofluorescence (IIF) slides, followed by confirmatory specific immunoassay for myeloperoxidase (MPO) or proteinase-3 (PR3) antibodies. These immunoassays are reportedly highly sensitive and specific for AAV diagnosis; however, this may vary depending on platform used. Due to locally observed cases of false-positive results, we sought to assess the performance characteristics of the ImmunoCAP fluorescent enzyme immunoassay (FEIA) for detection of MPO/PR3 antibodies. Method All positive MPO/PR3 antibody results via FEIA over a 5-year period were collated at Central Sydney Immunopathology Laboratory. We analysed electronic medical records to identify the associated clinical diagnosis for all cases, including those known to contribute to MPO/PR3 positivity, such as inflammatory bowel disease (IBD), cocaine use, and antithyroid medications. Potential confounding laboratory results, which may contribute to false-positive results (e.g., paraprotein, rheumatoid factor, anti-cardiolipin IgG) were also collected. We excluded cases where no clinical data were available, and cases where age was under 18 years. Local ethics board approval was granted for this study. Results Only 84 of 222 (37.8%) patients with a positive MPO or PR3 autoantibody had a diagnosis of AAV. Of the 138 cases without a diagnosis of AAV, 72 (52.2%) had IBD. All IBD cases had positive PR3 antibodies but were noted to have lower levels (mean = 8.8 U/mL) than those associated with AAV (mean = 30.0; p value <0.05). Only 11 PR3-positive IBD patients had a corresponding c-ANCA-positive pattern. We found 20% (n = 66) of cases had neither AAV nor IBD, of which 29 had a laboratory abnormality that may contribute to laboratory interference and false-positive results. Conclusion We found a high rate of false-positive results for MPO/PR3 antibodies via the ImmunoCAP FEIA in the real-world laboratory setting. Our results support the importance of correlating these results with IIF, clinical findings, and biopsy diagnosis and emphasise the need for clinician education to ensure ANCA testing is performed only where there is a high pre-test probability of AAV.