Analyzing Antineutrophil Cytoplasmic Antibody Associated Conditions in a Large, Random Sample of Patients: A Comparison of Enzyme-Linked Immunosorbent Assay and Indirect Immunofluorescence Tests

Aim: The aim of this study was to examine the prevalence of antineutrophil cytoplasmic antibodies (ANCA) in a diverse group of Turkish patients admitted to a tertiary care hospital, their clinical characteristics, and ANCA test results. Material and Methods: This retrospective study analyzed 10,726 serum samples tested for ANCA over 4.5 years. All patients requested ANCA indirect immunofluorescence (IIF) and enzyme-linked immunosorbent assay (ELISA) tests were included. Results: IIF/ELISA test results were positive for 973 (9.1%) patients, 875 IIF positive and 98 ELISA positive with negative IIF. Of the 875 IIF-positive patients, the p-ANCA pattern was observed in 631 (72.1%), and the c-ANCA pattern was observed in 222 (25.4%). MPO ELISA was positive in 16.6% (n=105), and PR3 ELISA was positive in 0.8% (n=5) of the p-ANCA positives, while MPO ELISA was positive in 0.9% (n=2), and PR3 ELISA was positive in 36.5% (n=81) of the c-ANCA positives. 11.6% (n=113) of the IIF/ELISA positives were ANCA-associated vasculitis (AAV). The ELISA test detected ANCA in 91 (80.5%) of the AAV patients. The positive predictive value (PPV) of IIF was 11.89%, while the PPV of the ELISA was 29.84%. The utilization of both IIF and ELISA resulted in a PPV of 41.75%. Conclusion: Antigen-specific immunoassays exhibit better diagnostic precision compared to IIF. Therefore, PR3- and MPO-ANCA ELISA are recommended for AAV screening. In patients with no suspicion of AAV, the more sensitive IIF test should be used. If IIF is positive, antigen-specific assays such as anti-MPO or anti-PR3 ELISA should confirm antigens.