Background: Chronic paranasal sinusitis (CPS) has been known as a surrogate marker for granulomatosis with polyangiitis (GPA). We investigated whether CPS at diagnosis may have an influence on the classification and outcomes of microscopic polyangiitis (MPA). Methods: We retrospectively reviewed the medical records of 106 immunosuppressive drug-naïve patients with MPA. We compared variables at diagnosis of MPA patients with CPS with either MPA patients without CPS or 29 GPA patients with CPS. We applied the algorithm for antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) proposed by the European Medicine Agency to 22 MPA patients with CPS and reclassify them. Death, relapse and end-stage renal disease were assessed as the poor outcomes. Results: Except for ENT manifestations, only pulmonary manifestation was more frequently observed in MPA patients with CPS than those without (77.3% vs 47.6%). No proteinase 3-ANCA was detected in all MPA patients with CPS. Meanwhile, general (63.6% vs 27.6%) and renal manifestations (81.8% vs 44.8%) more often developed in MPA patients with CPS than GPA patients with CPS. Of 22 MPA patients with CPS, 21 patients underwent biopsies. When CPS was not considered as a surrogate marker for GPA, all patients with CPS were reclassified as MPA. Ground glass opacity and reticulation on high-resolution computed tomography and renal vasculitis were helpful clues supporting the classification of MPA in patients with CPS. CPS at diagnosis was not associated with the outcomes of MPA. Conclusion: CPS might not be a sufficient surrogate marker for GPA in the classification of AAV.
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