Within-Day Variability in Key Assessment of SpondyloArthritis International Society Outcomes in Axial Spondyloarthritis

Background/Aims: This study aimed to investigate within-day variability in pain, stiffness, disease activity, physical function, spinal mobility, and other clinical outcomes in axial spondyloarthritis (axSpA) and identify predictors of within-day improvement.

Materials and Methods: In this single-center cross-sectional study, patients with axSpA underwent 2 standardized clinical evaluations on the same day: a morning session (9:00-10:00 am) and an afternoon session (2:00- 3:00 pm). Assessments included the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), Ankylosing Spondylitis Disease Activity Score incorporating C-reactive protein (ASDAS-CRP), Bath Ankylosing Spondylitis Metrology Index (BASMI), Ankylosing Spondylitis Quality of Life (ASQoL), Bath Ankylosing Spondylitis Functional Index (BASFI). Hip internal/external rotation range of motion was assessed using a goniometer, and chest expansion was measured using standardized methods. Multivariate logistic regression identified predictors of within-day improvement.

Results: Eighty patients with axSpA were included (mean age 44.3 ± 12.4 years; 63.7% male). Significant within-day improvements were observed in pain (P = .015) morning stiffness (P = .001), BASFI (P = .013), BASMI (P = .001), and ASQoL (P = .006), chest expansion (P = .032), and hip internal/external rotation (P = .005, P = .036, respectively) in the afternoon, whereas BASDAI and ASDAS-CRP remained stable. Higher morning BASDAI and lower body mass index independently predicted within-day improvement in BASFI. Non-smoking status was a strong predictor of ASQoL improvement.

Conclusion: Clinically meaningful within-day variability exists in key clinical parameters and patient-reported outcomes in axSpA. Assessment timing significantly influences outcome interpretation and should be standardized in both clinical practice and research to enhance reliability and comparability.

Cite this article as: Kilic G, Bayraktar Kaya S, Çelik B, Kilic E, Karkucak M. Within-day variability in key assessment of SpondyloArthritis International Society outcomes in axial spondyloarthritis. ArchRheumatol. 2026;41(4):434-441.