Development and psychometric evaluation of the PMR-Impact Scale: a new patient reported outcome measure for polymyalgia rheumatica

Objectives Polymyalgia rheumatica (PMR) causes pain, stiffness and disability in older adults. Measuring the impact of the condition from the patient’s perspective is vital to high-quality research and patient-centred care, yet there are no validated patient-reported outcome measures (PROMs) for PMR. We set out to develop and psychometrically evaluate a PMR-specific PROM.

Methods Two cross-sectional postal surveys of people with a confirmed diagnosis of PMR were used to provide data for field testing and psychometric evaluation. 256 participants completed the draft PROM. Distribution of item responses was examined and exploratory factor analysis and Rasch analysis were used to inform item reduction, formation of dimension structure and scoring system development. 179 participants completed the PROM at two-time points, along with comparator questionnaires and anchor questions. Test-retest reliability, construct validity and responsiveness were evaluated.

Results Results from the field-testing study led to the formation of the PMR-Impact Scale (PMR-IS), comprising four domains (symptoms, function, psychological and emotional well-being, and steroid side effects). Construct validity and test-retest reliability met accepted quality-criteria for each domain. There was insufficient evidence from this study to determine its ability to detect flares/deterioration, but the PMR-IS was responsive to improvements in the condition.

Conclusion The PMR-IS offers researchers a new way to assess patient-reported outcomes in clinical studies of PMR. It has been developed robustly, with patient input at every stage. It has good construct validity and test re-test reliability. Further work is needed to fully establish its responsiveness and interpretability parameters and assess its real-world clinical utility.