Predictive Validity and Responsiveness of Patient-Reported and Performance-Based Measures of Function in the Boston RISE Study

Assessing physical function is important for identifying older adults at risk for subsequent disability and other negative events such as hospitalization or falls and for evaluating treatment effects. The choice of functional measure for use as a primary outcome in studies of older adults is challenging; appropriate selection depends on the outcome of interest and evidence for its psychometric properties. There are two general approaches to measuring function: patient-reported and performance-based assessments. Patient-reported outcome measures (PROs) assess a patient’s perception of a broad array of functional activities while performance-based measures (PBMs) reflect an observer’s evaluation of a patient’s performance of specific physical tasks. The relative advantages of each approach have been debated (1,2). PBMs have been purported to offer better objectivity and validity, greater sensitivity to change and are thought to be less easily influenced by external factors. On the other hand, PROs offer low cost and convenience, and may represent a broader, more patient-centered assessment of functions applicable to an older person’s daily life. Studies comparing the psychometric properties of PROs and PBMs have yielded conflicting results, likely in part because previous comparisons have often been limited by measures that assess different aspects of function. The magnitudes of association between the two approaches have ranged from low to high (3,4). Early reports suggested that PROs lack sensitivity to change (5), however recent work supports the comparability of both approaches in specific clinical populations (1,6). Additionally, both PROs and PBMs of function predict subsequent disability among older adults (6,7), though studies directly comparing the predictive validity of the two approaches are scarce (8,9). The Short Physical Performance Battery (SPPB) and timed walking tests are among the most commonly used PBMs with strong evidence supporting their predictive value (7,10–13). Similarly, a wide variety of PROs have been published, though many have been limited by lack of conceptual clarity and concerns over their responsiveness (14). The Late-Life Function and Disability Instrument (LLFDI), a PRO of function and disability, was developed to address these limitations (15,16). Since its conception, the LLFDI has been used in studies involving more than 17,000 older adults with a wide range of health conditions (17). A recent systematic review (17) reported evidence supporting the LLFDI’s construct validity and sensitivity to change. However, few studies have evaluated this measure’s predictive validity and ability to detect change in functional status, nor included a comparison to PBMs of the same or similar aspects of function. This study aimed to determine the predictive validity and responsiveness of the Function Component of the Late-Life Function and Disability Instrument (LLFDI-FC) compared to commonly used PBMs of lower extremity function in older adults. We hypothesized that the LLFDI-FC would have similar predictive validity for adverse outcomes to the PBMs and that both types of measures would have comparable responsiveness to meaningful change in functional status over 2 years.