Patient Reported Outcome Measures for Rheumatoid Arthritis Disease Activity: Rasch measurement theory to identify items and domains

Objectives Disease Activity (DA) monitoring is a standard of care in Rheumatoid Arthritis (RA). There is demand for achieving this through Patient Reported Outcome Measures (PROMs). The aim of this study was to determine which items could be used to measure the construct of RA DA, by analysing legacy PROMs, using Rasch measurement theory (RMT) analyses. Methods Questionnaires including 10 legacy PROMs were sent to people with RA to create original and validation datasets. Items were grouped according to OMERACT domains and analysed using Principal Components Analysis. By domain RMT analyses in original dataset, and domain-level testlets were assessed to determine which measure the construct of RA DA. The result was then replicated in confirmatory factor analyses bifactor models and RMT analyses in the validation dataset. Psychometric properties of legacy PROMs was assessed in the original dataset. Results The total sample size was 691 (original: 398, validation: 293). The Patient Global domain was split into General health and Disease activity domains under RMT. General health and Fatigue domain items measure a separate construct to the construct of RA DA. A set of 12 Pain, Disease activity, Tenderness and swelling, Physical functioning and Stiffness domain items can be used to measure the construct of RA DA. No legacy PROMs fully fit the Rasch measurement model. Conclusion General health and Disease activity domain items are not interchangeable. 12 items form an item pool that can be used to measure the construct of RA DA. Legacy PROMs should not be recommended for use.