Application of the 2022 ACR/EULAR criteria for microscopic polyangiitis to patients with previously diagnosed microscopic polyangiitis

Objective This study applied the 2022 American College of Rheumatology/European Alliance of Associations for Rheumatology (the 2022 ACR/EULAR) criteria for microscopic polyangiitis (MPA) to patients with previously diagnosed MPA as per the 2007 European Medicines Agency algorithm (the 2007 EMA algorithm) and the 2012 revised International Chapel Hill Consensus Conference nomenclature of vasculitides (the 2012 CHCC definitions). The concordance rate between the new and old criteria was investigated. Methods This study included 117 patients with MPA, and the new criteria were applied to these patients. MPA can be classified when the total score is >= 5. Results The median age was 64.0 years. The concordance rate between the new and old criteria reached 96.6%. Four patients with previously diagnosed MPA were unclassified. Of these, three patients without myeloperoxidase (MPO)-antineutrophil cytoplasmic antibody (ANCA) (or perinuclear [P]-ANCA) were not reclassified as having MPA according to the new criteria, despite histopathological findings that were suggestive of MPA based on both the 2007 EMA algorithm and the 2012 CHCC definitions. Conversely, three of four patients with both MPO-ANCA (or P-ANCA) and proteinase 3 (PR3)-ANCA (or cytoplasmic [C]-ANCA) were reclassified as having both MPA and granulomatosis with polyangiitis (GPA) simultaneously according to the 2022 ACR/EULAR criteria for MPA and GPA. Conclusion In the new criteria, excessively high score was assigned to MPO-ANCA (or P-ANCA) and MPA-specific histopathological findings were not considered. Hence, the 2007 EMA algorithm and the 2012 CHCC definitions can be applied as additional criteria to complex cases.