Prevalence and Clinical Characteristics of Patients With Pseudogout and Concomitant Monosodium Urate Crystals

Objective: Pseudogout and gout are crystal-induced arthritides caused by calcium pyrophosphate dihydrate (CPPD) and monosodium urate (MSU) crystals. Reported coexistence rates are generally low (<6%), but prior studies mainly evaluated CPPD deposition in gout cohorts. The prevalence and clinical relevance of concomitant MSU crystals in pseudogout remain unclear. We aimed to determine the coexistence rate of CPPD and MSU crystals in patients with pseudogout and compare clinical characteristics and outcomes by coexistence status. Methods: This single-center retrospective cohort study was conducted at a tertiary emergency medical center in Tokyo. Consecutive patients diagnosed with pseudogout between April 2018 and November 2025 who underwent synovial fluid aspiration were included. CPPD crystals were identified by polarized light microscopy with simultaneous MSU assessment. Patients were classified into pseudogout-only and gout-superimposed groups. Clinical features, treatments, and time to improvement were compared between the two groups. Results: CPPD crystals were identified in 107 patients: median age, 86 years; interquartile range, 80-90 years; female sex, 79 (74%). Most patients had monoarticular disease, and the knee was most frequently involved in 92 patients (86%). MSU crystals were detected in 17 patients (15.9%), while 90 (84.1%) had pseudogout alone. Except for higher serum uric acid levels in the gout-superimposed group, no significant differences were found between groups in clinical characteristics, treatment, or outcomes. In coexistence cases, CPPD crystals predominated on polarized light microscopy. Conclusions: MSU crystal coexistence was observed in 15.9% of patients with crystal-proven pseudogout, exceeding previously reported rates. Systematic assessment for MSU crystals during synovial fluid microscopy may facilitate recognition of concomitant gout in patients with pseudogout; however, prospective studies are needed to determine whether such identification changes clinical management or improves patient outcomes.