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Introduction: Axial spondyloarthritis (axSpA) is a chronic inflammatory rheumatic disease characterized by back pain, functional impairment and reduced health-related quality of life. Disease activity is commonly assessed using objective inflammatory markers such as C-reactive protein (CRP) and patient-reported outcomes, including the axSpA Disease Activity Score (ASDAS) or the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI). Impairments in physical function is evaluated using the Bath Ankylosing Spondylitis Functional Index (BASFI). Vitamin D plays a key role in bone metabolism and has immunomodulatory properties that may influence inflammatory pathways in axSpA. While previous studies have explored the relationship between vitamin D deficiency and disease activity, findings have been inconsistent. Furthermore, the relationship between vitamin D status and treatment regimens in axSpA has not been well defined. The aim of the study was to investigate the association between serum vitamin D levels, disease activity, functional impairment and treatment regimens in axSpA patients in daily practice.

Methods: We assessed age, body mass index (BMI), serum vitamin D levels, HLA-B27 status, disease activity and functional impairment parameters in 38 patients diagnosed with axSpA in an outpatient department of a tertiary university hospital in 2025. Patients were stratified into groups based on disease activity (high disease activity BASDAI ≥ 4), functional impairment (high functional impairment (BASFI > 5)) and inflammation (elevated (CRP ≥ 0.5 mg/dl). Current therapeutic regimens were recorded. Patients were categorized into two treatment groups: those receiving biologic therapy/targeted synthetic Disease-Modifying Anti-Rheumatic Drug as well as non-steroidal anti-inflammatory drugs (NSAIDs) and those receiving NSAIDs only. Independent sample tests were used to compare clinical and laboratory parameters between groups and a p-value < 0.05 was considered statistically significant.

Results: The mean age of the patients was 44.4 ± 15.9 years (range 22–79 years), including 15 women and 23 men. The mean BMI was 27.4 ± 5.1 kg/m² (range 20–48) and 23 patients (62.2%) were HLA-B27 positive. Elevated CRP was observed in 14 patients. High functional impairment (BASFI > 5) was present in 20 (52.6%) patients and high disease activity (BASDAI ≥ 4) in 28 (73.7%) patients. The mean serum vitamin D level was 23.5 ± 13.2 ng/ml (range 5–58). 20 patients (52.6%) had low vitamin D (<20 ng/ml). Patients with elevated CRP had significantly lower mean vitamin D levels compared to those with normal CRP (14.9 vs 29.0 ng/ml, p=0.001). The mean vitamin D levels were lower in patients with high disease activity according to BASDAI compared to those with low disease activity (24.3 vs 28.8 ng/ml), but this difference was not statistically significant. Similarly, patients with high functional impairment according to BASFI had lower mean vitamin D levels compared to those with low BASFI scores (23.1 vs 27.3 ng/ml) without statistical significance (p=0.4). Twelve patients received biologic therapy, while 26 patients were treated with NSAIDs only.

Conclusion: In this cohort of patients with axSpA, lower vitamin D levels were significantly associated with elevated CRP, but not with patient-reported disease activity or functional impairment measured. This accounts for the effect of systemic inflammatory activity and the increased risk of for development of osteoporosis, despite the tendency to new bone formation which is characteristic for axSpA. The high prevalence of vitamin D insufficiency observed supports routine assessment of vitamin D status in axSpA.

Disclosures: None declared