Objectives: When ascertaining the occurrence of disability, long assessment intervals may be problematic because they do not account for the possibility of recovery or for deaths or losses to follow-up. Our objective was to compare the rates of disability obtained from single follow-up assessments with those obtained from monthly assessments for intervals up to 24 months.
Design: Prospective cohort study.
Setting: General community.
Participants: Seven hundred fifty-four nondisabled persons, aged 70 and older, categorized into three groups according to their risk for disability (low, intermediate, and high).
Measurements: Participants were followed with monthly telephone interviews to determine the occurrence of disability in one or more of four key activities of daily living (ADLs).
Results: For each risk group, the rates of disability obtained from monthly assessments of ADL function (cumulative disability) were considerably greater than those obtained from single follow-up assessments (prevalent disability). These differences in rates increased progressively as the length of the assessment interval increased. For example, the cumulative and prevalence rates of disability in participants in the intermediate risk group were 0.24 and 0.11 at 6 months, 0.36 and 0.20 at 12 months, 0.46 and 0.16 at 18 months, and 0.53 and 0.20 at 24 months, respectively. Although the overall rates were lower, the results for persistent disability, defined as a new disability that was present for at least 2 consecutive months, were similar. Although these differences in rates were attributable almost exclusively to recovery from disability in the first 6 months, they were due increasingly to deaths and losses to follow-up over the next 18 months, particularly in participants in the high-risk group.
Conclusions: The occurrence of disability is substantially underestimated by longitudinal studies with long assessment intervals. More frequent assessments of functional status could lead to an improved understanding of the course and overall burden of disability in community-living older persons.