Differential item functioning of the Arm function in Multiple Sclerosis Questionnaire (AMSQ) by language, a study in six countries.

Background: The Arm function in Multiple Sclerosis Questionnaire (AMSQ) has been developed as a self-reported measure of arm and hand functioning for patients with multiple sclerosis (MS). The AMSQ was originally developed in Dutch and to date translated into five languages (i.e. English, German, Spanish, French, and Italian).

Objective: The aim of this study was to evaluate differential item functioning (DIF) of the AMSQ in these languages.

Methods: We performed DIF analyses, using "language" as the polytomous group variable. To detect DIF, logistic regression and item response theory principles were applied. Multiple logistic regression models were evaluated. We used a pseudo value of 0.02 or more as the DIF threshold.

Results: A total of 1733 male and female patients with all subtypes of MS were included. The DIF analysis for the whole dataset showed no uniform or non-uniform DIF on any of the 31 items. All values were below 0.02.

Conclusion: The AMSQ is validated in six languages. All items have the same meaning to MS patients in Dutch, English, German, Spanish, French, and Italian. This validation study enables use of the AMSQ in international studies, for monitoring treatment response and disease progression.