The Rivermead Mobility Index allows valid comparisons between subgroups of patients undergoing rehabilitation after stroke who differ with respect to age, sex, or side of lesion.

Objective: To investigate differential item functioning or item bias of the Rivermead Mobility Index (RMI) and its impact on the drawing of valid comparisons with the RMI between subgroups of patients after stroke who differ with respect to age, sex, or side of lesion.

Design: Cross-sectional study.

Setting: A rehabilitation center in the Netherlands and 2 stroke rehabilitation units and the wider community in the United Kingdom.

Participants: The RMI was completed for patients undergoing rehabilitation after stroke (N=620; mean age ± SD, 69.2±12.5y; 297 [48%] men; 269 [43%] right hemisphere lesion, and 304 [49%] left hemisphere lesion).

Interventions: Not applicable.

Main Outcome Measures: Mokken scale analysis was used to investigate differential item functioning of the RMI between subgroups of patients who differed with respect to age (young vs older), sex (men vs women), and side of stroke lesion (right vs left hemisphere).

Results: No differential item functioning was found for any of the comparison subgroups.

Conclusions: The RMI allows valid comparisons to be made between subgroups of patients undergoing rehabilitation after stroke who differ with respect to age, sex, or side of lesion.