Underestimating physical function gains: comparing FIM motor subscale and interRAI post acute care activities of daily living scale.

Objective: To investigate the construct validity of the activities of daily living (ADLs) sections of 2 major systems developed to measure functional ability in rehabilitation settings. Health assessment systems can inform care planning as well as policy decision-making on service effectiveness. Frailty, comorbidity, and heterogeneity make it difficult to accurately measure health outcomes for older adults. Objective investigation of the value of geriatric rehabilitation services requires assessment systems that are comprehensive, reliable, valid, and sensitive to clinically relevant changes in older patients.

Design: Trained health care workers assessed patients with both tools at admission and discharge. We used Rasch analysis to compare the instruments' dimensionality, item difficulty, item fit, differential item function, and number of response options.

Setting: Musculoskeletal and geriatric rehabilitation units in 2 Ontario hospitals.

Participants: Older adults receiving rehabilitation (N=209; mean age ± SD, 78.5±9.3; 67% women).

Interventions: Not applicable.

Main Outcome Measures: FIM and the interRAI Post Acute Care Assessment (interRAI PAC).

Results: For both the FIM motor and the interRAI PAC ADLs items, the difficulty level of the items was much lower than the participant's level of ability, resulting in a large ceiling effect. Also, on both scales, less actual change in functional ability was required to move between the midlevel response options.

Conclusions: Both scales have limited ability to discriminate between subjects with higher functional ability, which indicates that they may underestimate the effectiveness of inpatient rehabilitation for this group of patients when used alone.