Real-World Concordance of Indirect Immunofluorescence and Antigen-Specific Assays in Antineutrophil Cytoplasmic Antibody Testing: A Retrospective High-Volume Laboratory Study.

BACKGROUND: Current guidelines recommend myeloperoxidase (MPO)- and proteinase 3 (PR3)-based immunoassays as the primary approach for antineutrophil cytoplasmic antibody (ANCA) testing, while indirect immunofluorescence (IIF) is reserved for selected clinical contexts. However, real-world data regarding concordance and workflow implications of these approaches remain limited. METHODS: In this retrospective laboratory-based study, ANCA results from 8976 consecutive patients evaluated in a high-volume tertiary laboratory were analyzed. Indirect immunofluorescence patterns and MPO/ PR3 reactivity were assessed simultaneously using an integrated IIF-based platform. Concordance between IIF patterns and antigen-specific reactivity was evaluated, and retrospective workflow simulations were performed to estimate the effects of MPO/PR3-first and IIF-first testing approaches on laboratory test utilization. RESULTS: Overall IIF positivity was observed in 589 patients (6.56%), whereas MPO and/or PR3 reactivity was detected in 307 patients (3.42%; 95% CI: 3.06-3.82%). Among IIF-positive patients, 438 (74.4%) showed no detectable MPO or PR3 reactivity, while 156 antigen-reactive patients (50.8%; 95% CI: 45.1-56.5%) showed no identifiable IIF reactivity. Retrospective workflow simulations suggested that an MPO/PR3-first approach would substantially reduce IIF testing volume, whereas an IIF-first strategy would markedly decrease antigenspecific testing volume. However, each approach was associated with distinct discordance patterns between IIF and MPO/PR3 reactivity within the study cohort. CONCLUSION: Substantial discordance was observed between IIF patterns and MPO/PR3 reactivity. The findings reflect laboratory concordance and workflow characteristics rather than true clinical diagnostic performance, and the clinical significance of discordant findings requires further evaluation in clinically characterized cohorts. Cite this article as: Arslan A, Togay A. Real-world concordance of indirect immunofluorescence and antigen-specific assays in antineutrophil cytoplasmic antibody testing: A retrospective high-volume laboratory study. Eurasian J Med. 2026, 58(4), 1498, doi: 10.5152/eurasianjmed.2026.261498.