Test-retest reliability, construct validity and responsiveness of the Polymyalgia Rheumatica Impact Scale (PMR-IS)

Problem

Polymyalgia rheumatica (PMR) causes pain, stiffness and disability in older adults. It usually has a sub-acute onset and responds rapidly to treatment with steroids, although the initial improvement is typically followed by longer periods of lower-level symptoms and episodes of relapse. Steroids themselves cause significant morbidity and adverse effects have to be balanced against PMR symptoms. There are currently no validated patient reported outcome measures (PROMs) that measure the impact of PMR and its associated treatments from the patient’s perspective .We developed the PMR-impact scale (PMR-IS) comprising symptom, function, emotional and psychological well-being and steroid side-effects domains and here we present its first validation study.

Approach

People diagnosed with PMR in the preceding 3 years, identified via primary and secondary care services in the West Midlands, were invited to participate. Participants completed a baseline questionnaire booklet comprising the PMR-IS, the modified Health Assessment Questionnaire (mHAQ) and the Short Form-36 (SF-36). A second questionnaire booklet, comprising a series of anchor questions and the PMR-IS, was completed 1-6 weeks later.Test-retest reliability was evaluated in the group reporting that they had ‘stayed the same’ on the anchor question. The intra-class correlation coefficient (ICCagreement), standard error of the measurement (SEM) and the limits of agreement (LoA) were calculated for each domain. Construct validity was assessed by testing pre-specified hypotheses about the relationship between the comparator questionnaires and domains of the PMR-IS.Responsiveness was evaluated by testing hypotheses about the mean change scores on domains of the PMR-IS in participants grouped according to their anchor question responses. Consideration was also given to the interpretability of the measure - risk of floor and ceiling effects, the smallest detectable change (SDC) and the minimally important change (MIC).

Findings

210 first booklets and 179 paired questionnaires were analysed. Test-retest reliability was good for each domain (ICC>0.8) although the LoA were fairly wide. Construct validity was satisfactory with only one out of 11 hypotheses rejected. Responsiveness and interpretability analysis are underway (results will be available for the meeting).

Consequences

This is the first PROM specifically developed for PMR and this study is the first step towards establishing it as a tool fit for use in research and clinical practice. The availability of such a measure will enable the assessment of what truly matters to people with PMR to be incorporated into research into the condition and ultimately improve person-centred care for PMR. The results from this study demonstrate good test-retest reliability and construct validity, satisfying COSMIN standards for these properties. Full results for other measurement properties will be presented at the meeting.