Validation of the Impact Index: can we measure disease effects on quality of life in patients with hip and knee osteoarthritis?

Purpose To validate the Impact Index, a short, publicly available scale that measures the extent to which a respondent’s health problem adversely impacts their quality of life. Methods Secondary analysis of patients with hip or knee osteoarthritis surveyed after visiting a surgeon at baseline ( N = 322) and about 6 months after the visit ( N = 283). Patients responded to the Impact Index and previously validated questionnaires about overall health, pain, and function. The Impact Index includes four questions that ask how much the respondent is bothered, worried, limited, or in pain due to their health condition over the past 30 days. Total scores range from 0 to 12; higher scores indicate more deleterious impact. Results Patients were mostly female (55%), majority white (95%), had an average age of 65 (SD = 9), and most had surgery (64%). The baseline Impact Index score was 9.48 (SD = 2.63); at follow up 4.75 (SD = 3.54). Impact Index was related to overall health at baseline ( r = − 0.49). For knee patients at baseline, Impact Index was negatively related to their knee symptoms ( r = − 0.49) and knee pain ( r = − 0.67). For hip patients at baseline, Impact Index was negatively related to the Harris Hip score ( r = − 0.62). Scale directions varied; however, the signs of all correlations were as hypothesized. The Impact Index was predictive of surgical choice ( p − 3.4) and Guyatt Responsiveness Statistics (> − 2.3). Conclusions The Impact Index demonstrated strong evidence of validity, reliability, and responsiveness in hip or knee osteoarthritis patients.