Likelihood ratios of ANCA fluorescence patterns and anti-MPO/PR3 antibody levels in ANCA-associated Vasculitis: A framework for refined diagnostic interpretation.

OBJECTIVES: To determine the likelihood ratios (LRs) of anti-neutrophil cytoplasmic antibody (ANCA) fluorescence patterns and myeloperoxidase (MPO)/proteinase 3 (PR3)-ANCA antibody levels in ANCA-associated vasculitis (AAV), and evaluate their implications for ANCA interpretation. METHODS: This retrospective study included a screening cohort of 38,610 patients, a diagnostic cohort of 226 newly diagnosed AAV patients and 738 disease controls, and an independent validation cohort of 116 AAV patients and 448 controls. Diagnostic performance was assessed using result-specific LRs. RESULTS: ANCA fluorescence patterns showed limited disease specificity when interpreted alone. In microscopic polyangiitis (MPA), MPO-ANCA yielded an LR+ of 31.74 (95% CI 20.76-48.52), increasing to 46.64 (27.68-78.59) when combined with P-ANCA. Formalin-fixed P-ANCA positivity provided limited incremental value beyond MPO-ANCA testing. In granulomatosis with polyangiitis (GPA), PR3-ANCA yielded an LR+ of 44.03 (25.16-77.03), while combined PR3-ANCA/C-ANCA positivity showed the highest diagnostic value (LR+ 542.20, 75.93-3872.01), although confidence intervals were wide. Antibody-level analyses demonstrated a progressive increase in LR+ with increasing antibody levels. In the combined MPA/GPA cohort, antigen-specific antibody levels ≥2× the assay cut-off were associated with LR+ values >10, with a similar level-dependent trend observed in the validation cohort. CONCLUSIONS: ANCA fluorescence patterns provide complementary diagnostic information, whereas formalin-fixed substrate testing adds limited value when reliable MPO-ANCA assays are available. An LR-based framework integrating antibody levels and fluorescence patterns may improve the interpretation of ANCA results in suspected AAV.