External validation of the polymyalgia rheumatica impact scale: a prospective cohort study.

Objectives: To externally validate the PMR impact scale (PMR-IS).

Methods: We conducted a prospective cohort study at the University Hospitals Leuven, Leuven, Belgium. Recently diagnosed PMR patients were included between July 2022 and December 2023 and followed until 1 year after diagnosis. All patients completed the PMR-IS, HAQ Disability Index, 36-item Short Form and a visual analogue scale for pain at every visit. Internal consistency, floor and ceiling effects, construct validity, responsiveness and discriminatory power for detecting relapse on the PMR-IS were assessed.

Results: Fifty-five PMR patients (mean age 71 years, 47% female) were included, who had a total of 246 visits. Internal consistency, construct validity and responsiveness met the quality criteria for the symptoms, function and emotional and psychological well-being subdomains. The internal consistency of the glucocorticoid side effects subdomain was insufficient and only one of the three hypotheses for construct validity were met. The function and emotional and psychological well-being subdomains showed a floor effect, while no ceiling effect was observed. The symptoms, function and emotional and psychological well-being subdomains had a good discriminatory power for detecting relapse [area under the curve (AUC) 0.89, 0.86 and 0.72, respectively], but the PMR activity score performed better (AUC 0.94,  < 0.05 for all subdomains).

Conclusion: This study validates the good measurement properties of the symptoms, function and emotional and psychological well-being subdomains of the PMR-IS. In contrast, the glucocorticoid side effects subdomain did not show adequate internal consistency and construct validity, necessitating further validation and possibly refinement of its items prior to application in clinical trials or daily practice.