Construct validation of a knee-specific functional status measure: a comparative study between the United States and Israel.

Background: Comparative effectiveness research (CER) requires valid outcome measures that discriminate patients by risk factors in similar ways across settings. Standardized functional status (FS) measures in physical therapy are used routinely in multiple countries, creating the potential for CER among countries.

Objective: The purpose of this study was to assess known-groups construct validity of a knee-specific FS measure within and between 2 countries for patients receiving outpatient physical therapy due to knee impairments.

Design: This was a longitudinal, observational cohort study.

Methods: The participants were 4,972 and 2,964 adult (age ≥18 years) patients with knee impairments from Israel and the United States, respectively. Differences in patient characteristics between the 2 countries were assessed using chi-square statistics and 2-sample t tests, as appropriate. Known-groups validity within and between the countries was assessed using 2-way analysis of covariance predicting FS at discharge, with sex, age, symptom acuity, surgical and exercise history, intake medication use, and country as risk-adjustment factors. Intake FS was the covariate. To compare how FS discriminated patient groups between countries, each factor was tested separately with models including an interaction term between the factor and country.

Results: Patients were different between countries but had similar discharge FS trends, including: higher outcomes in patients who were male, were younger, had acute conditions, had one surgical procedure related to their knee impairment, were more physically active, and did not use related medication at admission. Interactions were not significant for sex, symptom acuity, and exercise history but were significant for age, surgical history, and medication use. Limitations Although strict patient selection criteria were set, some patient selection bias still might have existed.

Conclusions: The results demonstrated the knee FS measures would be valid for use in CER between Hebrew-speaking patients (Israel) and English-speaking patients (United States).