Refining the conceptual basis for rehabilitation outcome measurement - Personal core and instrumental activities domain

Background: Rehabilitation outcome measures routinely include content on performance of daily activities; however, the conceptual basis for item selection is rarely specified. These instruments differ significantly in format, number, and specificity of daily activity items and in the measurement dimensions and type of scale used to specify levels of performance. We propose that a requirement for upper limb and hand skills underlies many activities of daily living (ADL) and instrumental activities of daily living (IADL) items in current instruments, and that items selected based on this definition can be placed along a single functional continuum. Objective: To examine the dimensional structure and content coverage of a Personal Care and Instrumental Activities item set and to examine the comparability of items from existing instruments and a set of new items as measures of this domain. Methods: Participants (N = 477) from 3 different disability groups and 4 settings representing the continuum of postacute rehabilitation care were administered the newly developed Activity Measure for Post-Acute Care (AM-PAC), the SF-8, and an additional setting-specific measure: FIM (in-patient rehabilitation); MDS (skilled nursing facility); MDS-PAC (postacute settings); OASIS (home care); or PF-10 (outpatient clinic). Rasch (partial-credit model) analyses were conducted on a set of 62 items covering the Personal Care and Instrumental domain to examine item fit, item functioning, and category difficulty estimates and unidimensionality. Results: After removing 6 misfitting items, the remaining 56 items fit acceptably along the hypothesized continuum. Analyses yielded different difficulty estimates for the maximum score (eg, "Independent performance") for items with comparable content from different instruments. Items showed little differential item functioning across age, diagnosis, or severity groups, and 92% of the participants fit the model. Conclusions: ADL and IADL items from existing rehabilitation outcomes instruments that depend on skilled upper limb and hand use can be located along a single continuum, along with the new personal care and instrumental items of the AM-PAC addressing gaps in content. Results support the validity of the proposed definition of the Personal Care and Instrumental Activities dimension of function as a guide for future development of rehabilitation outcome instruments, such as linked, setting-specific short forms and computerized adaptive testing approaches.