Background: Mobility is a key indicator of physical functioning in older people, but there is limited evidence of the reliability of mobility measures in older people with cognitive impairment. This study aimed to examine the test-retest reliability and measurement error of common measurement instruments of mobility and physical functioning in older patients with dementia, delirium or other cognitive impairment.
Methods: A cross-sectional study was performed in a geriatric hospital. Older acute medical patients with cognitive impairment, indicated by a Mini-Mental State Examination (MMSE) score of ≤24 points, were assessed twice within 1 day by a trained physiotherapist. The following instruments were applied: de Morton Mobility Index, Hierarchical Assessment of Balance and Mobility, Performance-Oriented Mobility Assessment, Short Physical Performance Battery, 4-m gait speed, 5-times chair rise test, 2-min walk test, timed up and go test, Barthel Index mobility subscale and Functional Ambulation Categories. As appropriate, the intraclass correlation coefficient (ICC), Cohen's kappa, standard error of measurement, limits of agreement and minimal detectable change (MDC) values were estimated.
Results: Sixty-five older acute medical patients with cognitive impairment participated in the study (mean age: 82 ± 7 years; mean MMSE: 20 ± 4, range: 10 to 24 points). Some participants were physically or cognitively unable to perform the gait speed (46%), 2-min walk (46%), timed up and go (51%) and chair rise (75%) tests. ICC and kappa values were above 0.9 in all instruments except for the gait speed (ICC = 0.86) and chair rise (ICC = 0.72) measures. Measurement error is reported for each instrument. The absolute limits of agreement ranged from 11% (de Morton Mobility Index and Hierarchical Assessment of Balance and Mobility) to 35% (chair rise test).
Conclusions: The test-retest reliability is sufficient (> 0.7) for group-comparisons in all examined instruments. Most mobility measurements have limited use for individual monitoring of mobility over time in older hospital patients with cognitive impairment because of the large measurement error (> 20% of scale width), even though relative reliability estimations seem sufficient (> 0.9) for this purpose.
Trial registration: German Clinical Trials Register ( DRKS00005591 ). Registered 2 February 2015.
Keywords: Cognitive impairment; Dementia; Limits of agreement; Measurement error; Minimal detectable change; Mobility limitation; Older people; Outcome measure; Reliability.