Background The Stanford Health Assessment Questionnaire Disability Index (HAQ-DI) is a widely used patient reported outcome for functional disability measurement in rheumatoid arthritis (RA). Minimal clinically important differences (MCIDs) have previously been calculated based on the original ordinal HAQ-DI scores resulting in a dependence of the MCIDs on baseline scores (1). This causes limited generalizability and validity of the proposed MCIDs. The Rasch measurement model provides a transformation of these ordinal scores to an underlying latent scale with interval scale properties. This requires adequate fit of HAQ-DI data to the model.
Objectives 1) To examine the internal construct validity of the Danish version of the HAQ-DI (scored without correction for devices) and 2) to determine a MCID of the HAQ-DI in a cohort of RA patients initiating anti-rheumatic treatment in clinical practice based on the transformed linear logit scale of the HAQ-DI.
Methods RA patients registered in the DANBIO registry at the out-patient clinic at Center for Rheumatology and Spine Diseases, Copenhagen, Denmark were included in the study if HAQ-DI and Patient Global Visual Analogue Scores (PtGl) scores were available at a baseline visit and after > three months of follow-up after initiation of a) first synthetic Disease Modifying Anti Rheumatic Drug (sDMARD) (if disease duration <12 months) or 2) first biological drug (if disease duration >12 months). The Rasch model was fitted to HAQ-DI data at baseline and follow-up and item fit evaluated in separate analyses by comparing observed and expected item-restscore correlations (2). MCID was calculated as the median changes of A) the original and B) logit HAQ-DI score in the group of patients who had improved by 15–30 mm (ie. minimal improvement) on a 0–100 mm PtGl scale.
Results 362 RA patients were included in the study (Table 1). HAQ-DI data showed acceptable fit to the Rasch model as no significant evidence of misfit was disclosed at the first time-point and only a single misfitting item (the “Grip” sub-item, p=0.01) was identified at the second time-point. Consistent item ranking across time indicated instrument invariance. Changes in ordinal and logit HAQ-DI scores were strongly correlated (Spearmans rho =0.935, p<0.001), but the association between them is not completely linear. Sixty-one patients had an improvement > MCID on the logit scale (0.48), but no improvement on the ordinal scale (MCID 0.250), while no patients had the opposite pattern.
Conclusions The Danish version of the HAQ-DI scored without correcting for devices showed acceptable internal construct validity and thus a MCID based on a linear transformed scale could be calculated for this study (0.48). Application of the logit MCID classified an additional 17% of patients as having achieved a MCID compared to the MCID calculated on the ordinal scale. This finding has potential implications for the use of ordinal-based MCID and the powering of future studies.
Doganay Erdogan B, Leung YY, Pohl C, Tennant A, Conaghan PG. J Rheumatol. 2016 Jan;43(1):194–202
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Disclosure of Interest None declared