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Objectives: Axial spondyloarthritis (axSpA) displays varying disease activity over time. However, few studies have examined the association between disease activity and seasonal differences, and consequently, evidence of a seasonality in disease activity in axSpA is limited. We therefore aimed to study the variation in measures of disease activity, perceived well-being, and functional ability, including both patient-reported and objective measures throughout the year, and their relationship with seasons.

Methods: Objective and patient-reported disease parameters from 44 987 assessments on 5197 individuals were retrieved from the Swedish Rheumatology Quality register. The variation in different measures of disease activity over the year was examined using generalized additive models with cyclic cubic splines.

Results: Patient reported and objective measures of disease were found to vary congruently over the year, with a trend of decreased disease parameters in the spring. Mean CRP values showed a statistically significant variation throughout the year, with highest in July (7.35 mg/L) and lowest in March (6.88 mg/L). Significant seasonal variation was also seen in BASDAI (range of mean values 3.67–3.72), BASFI (2.79–2.87) and BASMI (3.26–3.31), with highest values in late summer, beginning of fall, or in the fall, respectively. In subgroup analysis, significant variation was only seen in subjects with a disease duration of more than 10 years.

Conclusion: Although a statistically significant seasonal variation in disease parameters was found, the absolute variation was close to none and may therefore be clinically irrelevant.