Abstract
BACKGROUND: Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) has improved survival outcomes with advancements in immunosuppressive therapies; however, the increased incidence of infections remains a significant concern. Reports on infection risk factors other than age are limited, especially in older adults with AAV who are at a higher risk. METHODS: We aimed to identify the risk factors for early-phase infection during treatment initiation in older adults (aged ≥ 75 years) with myeloperoxidase (MPO)-ANCA-positive AAV (MPO-AAV). This was a single-center, retrospective observational study that included 50 patients who were classified as having microscopic polyangiitis. Severe infections were defined as those requiring hospitalization within six months of treatment initiation. RESULTS: Severe infections occurred in 17 (34%) patients. No statistically significant associations were observed between disease incidence and either combined immunosuppressive therapy or organ-specific lesions. However, trimethoprim-sulfamethoxazole (TMP-SMX) was significantly associated with a reduced risk of infection. This association remained statistically significant after adjustment for sex (hazard ratio, 0.23; 95% CI, 0.08–0.64; P < 0.01). CONCLUSIONS: In older adults with MPO-AAV, TMP-SMX use was associated with a reduced risk of early severe infection. Although limited by the observational nature of the study, these findings suggest a potential role for TMP-SMX prophylaxis in high-risk elderly populations. CLINICAL TRIAL NUMBER: Not applicable. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12882-025-04695-y.