Vitamin D status in patients with arthritis and osteoporosis: a comparative analysis of inpatient and outpatient cohorts.

Background: Vitamin D deficiency is a global health issue linked to musculoskeletal disorders, including osteoporosis and arthritis. Despite its critical role in bone and immune health, large-scale comparisons of vitamin D status between outpatient and inpatient populations remain limited. Objective: This study aimed to compare vitamin D levels in patients with and without arthritis or osteoporosis across outpatient and inpatient settings and evaluate their associations with clinical outcomes. Methods: We performed a retrospective cross-sectional analysis of 37800 adults treated at the Department of Orthopedics in Nanjing Drum Tower Hospital from 2015 to 2020. Serum 25(OH)D concentrations were categorized as normal, low, or deficient. Patients were stratified by diagnosis (arthritis and osteoporosis) and care setting (outpatient vs. inpatient). Statistical comparisons were made using t-tests, Chi-square tests, and log-binomial regression adjusted for age, sex, the alternate musculoskeletal diagnosis, and osteocalcin. Diabetes mellitus was only consistently available for inpatients and was therefore handled as a descriptive inpatient characteristic rather than in the outpatient characteristic and regression models.Adjusted relative risks (RRs) for arthritis and osteoporosis were estimated according to vitamin D status, with normal vitamin D as the reference category, using log-binomial regression adjusted for age, sex, osteocalcin, and the alternate musculoskeletal diagnosis where applicable. Results: Suboptimal vitamin D status (low or deficient levels) was widespread across both settings, affecting 52% of outpatients and 41% of inpatients. The overall mean 25(OH)D was 26.25 ± 7.14 ng/mL in outpatients and 27.38 ± 6.58 ng/mL in inpatients. Among outpatients, those with arthritis exhibited higher deficiency rates (16.2% vs. 13.6% in non-arthritis) and significantly elevated arthritis risks (RR ranging from1.24 to 1.32). Among inpatients, however, the arthritis analysis was limited by the small number of cases (n=176) and wide confidence intervals (RR for deficiency=1.08, 95% CI 0.65-1.70), so the finding is inconclusive and does not support the association. Deficiencies were particularly pronounced in osteoporosis cases: 26.3% of osteoporotic outpatients were deficient (vs.12.8% controls; RR = 2.79), while inpatients with osteoporosis also showed higher deficiency (13.7% vs. 10.2%; RR = 1.50), with both differences statistically significant (p<0.001). Conclusion: Vitamin D deficiency was common, particularly among osteoporosis patients in outpatient settings. Associations were strongest for outpatient osteoporosis, while the inpatient arthritis estimates were limited by the small number of cases. These findings suggest that targeted assessment of vitamin D status in patients with osteoporosis or arthritis, rather than universal screening, may help identify individuals at higher risk of deficiency and related musculoskeletal complications.