Development and evaluation of a crosswalk between the SF-36 physical functioning scale and Health Assessment Questionnaire disability index in rheumatoid arthritis.
BACKGROUND
The SF-36 physical functioning scale (PF-10) and the Health Assessment Questionnaire disability index (HAQ-DI) are the most frequently used instruments for measuring self-reported physical function in rheumatoid arthritis (RA). The objective of this study was to develop a crosswalk between scores on the PF-10 and HAQ-DI in RA.


METHODS
Item response theory (IRT) methods were used to co-calibrate both scales using data from 1791 RA patients. The appropriateness of a Rasch-based crosswalk was evaluated by comparing it with crosswalks based on a two-parameter and a multi-dimensional IRT model. The accuracy of the final crosswalk was cross-validated using baseline (n = 532) and 6-month follow-up (n = 276) data from an independent cohort of early RA patients.


RESULTS
The PF-10 and HAQ-DI adequately fit a unidimensional Rasch model. Both scales measured a wide range of functioning, although the HAQ-DI tended to better target lower levels of functioning. The Rasch-based crosswalk performed similarly to crosswalks based on the two-parameter and multidimensional IRT models. Agreement between predicted and observed scale scores in the cross-validation sample was acceptable for group-level comparisons. The longitudinal validity in discriminating between disease response states was similar between observed and predicted scores.


CONCLUSIONS
The crosswalk developed in this study allows for converting scores from one scale to the other and can be used for group-level analyses in patients with RA.
Department of Psychology, Health and Technology, University of Twente, Enschede, The Netherlands. p.m.tenklooster@utwente.nl.
Peter M
PM
ten Klooster
Martijn A H
MA
Oude Voshaar
Barbara
B
Gandek
Matthias
M
Rose
Jakob B
JB
Bjorner
Erik
E
Taal
Cees A W
CA
Glas
Piet L C M
PL
van Riel
Mart A F J
MA
van de Laar
D000328:Adult; D000368:Aged; D001172:Arthritis, Rheumatoid / Q000503:physiopathology*; D016001:Confidence Intervals; D004185:Disability Evaluation; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D008962:Models, Theoretical; D009426:Netherlands; D011788:Quality of Life; D011795:Surveys and Questionnaires / Q000592:standards*