Axial spondyloarthritis patients with comorbid fibromyalgia feel worse, work less and more often try multiple biological therapies: results from a population-based, cross-sectional study investigating the discriminative capacity of pressure algometry

To estimate prevalence and associated factors of comorbid fibromyalgia in axial spondyloarthritis (axSpA), and to explore discriminative capacity (for discerning fibromyalgia) of algometry-assessed pain sensitivity measures. AxSpA patients from the population-based SPARTAKUS cohort evaluated for fibromyalgia (1990 ACR criteria) were included (n = 243; r-axSpA/nr-axSpA = 165/78). Factors associated with fibromyalgia were cross-sectionally analyzed by logistic/linear regression, and discriminative potential of algometry-assessed measures estimated by ROC-curve analysis. A fibromyalgia prevalence of 9% was demonstrated, with a higher frequency in female than male patients (17%/2%;adjusted p < 0.001) and more than a doubling in frequency per 5-unit BMI increase (adjusted p < 0.002). Fibromyalgia was associated with several disease/work outcomes, including higher disease activity, lower quality-of-life, and less employment although without association to swollen joints/CRP. Additionally, patients with (versus without) fibromyalgia had higher bDMARD exposure (19%/5% had tried ≥ 3 bDMARDs; adjusted p = 0.009), and were more often on opioids (62%/16%;adjusted p < 0.001). Algometry-assessed measures displayed a sensitivity/specificity of 79%/70% (pain threshold) and 71%/83% (pain tolerance) for fibromyalgia differentiation (positive/negative predictive values 16%/99% and 23%/98%, respectively). Fibromyalgia is a frequent comorbidity in axSpA, more common in female patients/patients with higher BMI, and associated with worse levels of patient-reported disease/work outcomes. Our findings highlight the challenge of assessing axSpA disease activity when fibromyalgia is present and suggest a role for algometry as a complementary, evaluator-independent assessment tool, potentially useful for fibromyalgia rule-out but less so for rule-in. The results further suggest that fibromyalgia may be associated with DMARD overtreatment, reflecting a need for earlier and more precise targeting.