Testing measurement equivalence of the SF-36 questionnaire across patients on hemodialysis and healthy people.

Background: Differential item functioning (DIF) occurs when members from different groups respond differently to particular items in a health-related quality of life (HRQoL) questionnaire after controlling for underlying HRQoL construct. This study aimed to assess DIF in the SF-36 questionnaire and its effect on comparing HRQoL scores across patients on HD and healthy people.
Methods: One hundred fifty patients on maintenance hemodialysis (HD) and 642 healthy individuals filled out the Persian version of the SF-36 questionnaire. Multiple-group multiple-indicator multiple-causes (MG-MIMIC) model was used to assess DIF across patients on HD and healthy population.
Results: Sixteen out of 36 (44.4 %) items were flagged with DIF. Six out of 16 items (37.5 %) were flagged with uniform DIF, nine items (56.2 %) with non-uniform DIF, and one item (6.2 %) with both uniform DIF and non-uniform DIF. DIF items were associated with all subscales with the exception of the limitation due to physical problems and bodily pain subscales. The significant lower HRQoL scores of patients on HD in comparison with healthy people in the physical functioning and vitality subscales did not change after removing the items with uniform DIF.
Conclusions: Our findings revealed that patients on HD and healthy people perceived the meaning of the items in SF-36 questionnaire differently. Although the impact of DIF is minimal, the cross-group comparison across patients on HD and healthy people should be performed with caution.