Abstract
ObjectiveTo assess psychometric properties and cross-language measurement equivalence of six versions of the Bristol Rheumatoid Arthritis Fatigue Scale (BRAF-MDQ) and the Rheumatoid Arthritis Impact of Disease Score (RAID in rheumatoid arthritis (RA).MethodsBoth questionnaires were completed by French (n = 206), German (n = 206), Dutch (n = 317), Spanish (n = 157), Swedish (n = 170) and UK (n = 210) RA patients. The presence of cross-language differential item functioning (DIF) was examined using the generalized partial credit model. The impact of DIF on the item and total scores was examined by comparing DIF unadjusted and DIF adjusted expected item and scale scores. IRT-based methods were used to assess psychometric properties of the instruments.Results11 of the 20 BRAF-MDQ (55%) and 4 of the 7 RAID items (57%) exhibited significant DIF in at least one of the six countries. The mean number of items with DIF per country was 2.6 for BRAF-MDQ and 1.1 for RAID. However, the impact of DIF on the total RAID and BRAF-MDQ scores, as well as the BRAF subscales, was found to be negligible at the group level. Only for the BRAF physical subscale was there evidence of minor DIF. Marginal reliabilities of BRAF-MDQ (0.93) and RAID (0.89) were excellent, and precise scores could be obtained across the spectrum of disease impact and fatigue scores measured by these PROMs.ConclusionThis study supports the cross-language measurement equivalence of BRAF-MDQ and RAID and provides further support for the psychometric properties of these measures in RA.
Highlights
Rheumatoid arthritis is an inflammatory joint disease, often with a chronic course that is known to impact patients’ quality of life in a variety of ways
The results of the likelihood ratio test showed that the Generalized partial credit model (GPCM) fitted significantly better for both the BRAF-MDQ χ (DF 50) = 1175, p < 0.01) and Rheumatoid Arthritis Impact of Disease (RAID) models χ (DF 26) = 400, p < 0.01)
The BRAF item: “Have you been embarrassed because of your fatigue?” was most strongly affected by differential item functioning (DIF) according to the Lagrange Multiplier (LM) test for DIF across countries (LM = 127.46, p < 0.01, ES = 0.05) Inspection of the ES per country for this item showed that response behavior was different in France (ES = 0.06), Spain (ES = 0.08), and Sweden (ES = 0.05), but not in the remaining countries (ES = 0.03 in all cases), see
Summary
Rheumatoid arthritis is an inflammatory joint disease, often with a chronic course that is known to impact patients’ quality of life in a variety of ways. Patientreported outcomes such as pain and physical function have. A variety of measures has been developed to facilitate the measurement of such PROs. For example, a patient-reported response index, the Rheumatoid Arthritis Impact of Disease (RAID) score, which combines 7 PRO domains, including fatigue, emotional well-being and sleep quality in one measure is available and evidence regarding its measurement properties has been published [5, 6]. Several studies have supported its measurement properties in RA [7,8,9]
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