Sex Differences in Disease Activity Measures in Axial Spondyloarthritis and Their Association with Concomitant Fibromyalgia: A Retrospective Cross-Sectional Analysis of a Saudi Cohort

Background/Objectives: Axial spondyloarthritis (axSpA) was long considered a male disease, yet women often report higher disease activity; whether this reflects sex differences or comorbidity is unclear. In an under-studied Saudi cohort, we examined sex differences in disease activity and function and associations with fibromyalgia and body mass index (BMI). Methods: A retrospective cross-sectional analysis of de-identified baseline data included 160 adults who met the Assessment of SpondyloArthritis International Society (ASAS) criteria at a single center. Between-sex comparisons used Benjamini–Hochberg correction; sequential multivariable regression evaluated attenuation of the sex–disease activity association after covariate adjustment. Results: Of 160 patients, 56 (35%) were women. Women more often had non-radiographic disease, higher BMI, fibromyalgia, and elevated erythrocyte sedimentation rate (ESR) (all q < 0.05), and scored higher on the symptom-based Bath Ankylosing Spondylitis Disease Activity Index (BASDAI; median 5.65 [IQR 4.67–6.60] vs. 5.15 [4.30–5.72]; p = 0.007, q = 0.044). The Bath Ankylosing Spondylitis Functional Index (BASFI) was borderline (q = 0.052), and C-reactive protein (CRP)-anchored Ankylosing Spondylitis Disease Activity Score (ASDAS-CRP) did not differ significantly (median 3.70 [IQR 3.00–3.90] vs. 3.50 [3.00–3.80]; p = 0.099, q = 0.148). The female BASDAI difference was attenuated after adjustment for concomitant fibromyalgia. Higher ESR was partly attenuated after adjustment for BMI, whereas CRP did not differ by sex. In the analysis restricted to participants without fibromyalgia, between-sex estimates for BASDAI, BASFI, and ASDAS-CRP were smaller and imprecise. Conclusions: The attenuation of the female-sex association with BASDAI after adjustment for concomitant fibromyalgia should be interpreted as statistical attenuation only. Because fibromyalgia and BASDAI share symptom domains, particularly pain and fatigue, this attenuation may partly reflect overlapping symptom content and possible classification circularity rather than an explanatory or causal relationship.