Cross-Cultural Validity and Differential Item Functioning of the Orthotics and Prosthetics Users' Survey with Swedish and United States Users of Lower-Limb Prosthesis

Objectives To investigate the cross-cultural validity of the Orthotics and Prosthetics Users' Survey (OPUS), to evaluate differential item functioning (DIF) related to country, sex, age, amputation level, and amputated side (unilateral, bilateral), and to determine known-group validity of the OPUS. Design Survey. Setting Outpatient clinics. Participants The sample (N=321) consisted of Swedish (n=195) and U.S. (n=126) adults using lower-limb prostheses. Interventions Not applicable. Main Outcome Measures Four OPUS modules were used: lower extremity functional status, client satisfaction with device (CSD), client satisfaction with services (CSS), and health-related quality of life. Rasch analysis was used to calculate measures for persons and items. Results The cross-cultural validity was satisfactory. Many items demonstrated DIF related to country and demographic characteristics, but the impact on mean person measures was negligible. The rating scales of CSD and CSS needed adjustments, and the unidimensionality of CSD and CSS was weak. The differences between the mean measures of known patient groups were statistically significant for 2 out of 6 comparisons. Conclusions This study supports the validity of OPUS measure comparisons between Sweden and the United States and between subgroups with different demographic characteristics. Some of the country-related DIF may reflect the different health care financing systems. The findings demonstrate that the OPUS can discriminate between certain patient groups. The results also challenge some of our preconceptions about persons with bilateral amputation, indicating that we might know these persons less well than we think.