Discordance between self-reported and performance-based function among knee osteoarthritis surgical patients: Variations by sex and obesity

BACKGROUND: There is currently no standardized method for measuring functional status in knee osteoarthritis (OA) patients, despite that it is one of the top priorities when determining eligibility for total knee arthroplasty (TKA). The purpose of the current investigation was to identify factors associated with discordance between individual self-report and performance-based measures of function for obese and non-obese men and women with knee OA. METHODS: In a cohort of 727 knee OA patients scheduled for TKA, physical function prior to surgery was assessed with the self-reported physical function subscale of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC-pf), and the performance-based Timed Up and Go (TUG). Data on sociodemographic characteristics, health status, knee pain intensity, symptomatic joint site count, and pain catastrophizing were collected via questionnaire. The primary outcome was the difference in rescaled score between a participant’s self-report and performance-based measures of function. Multivariable linear regression stratified by sex and obesity status was used to identify factors associated with discordance. RESULTS: The mean age of participants was 65.5 years and 55% were women. With younger age, self-reported scores indicated increasingly worse function compared to performance-based scores, regardless of sex or obesity status. Among non-obese individuals, greater knee pain intensity was associated with a participant’s self-report score indicating increasingly worse function compared to their performance-based score. For obese women, pain catastrophizing, and number of symptomatic joints were also associated with discordance as was reporting fewer comorbidities. CONCLUSIONS: Physical function may be differentially represented by self-reported and performance-based measures depending on a variety of patient factors. Our findings add to the evidence which suggests both measures should be used when assessing functional status prior to TKA.