Evaluating change using patient-reported outcome measures in knee replacement: the complementary nature of the EQ-5D index and VAS scores.

PURPOSE: Using the UK National Health Service's Patient Reported Outcome Measures data, we examined the magnitude of changes and relationship among the EQ-5D index, EQ-5D Visual Analog Scale (EQ-VAS), and Oxford Knee Score (OKS) in patients undergoing knee replacement.
METHODS: Patients undergoing knee replacements in 2009-2011 completed the EQ-5D and OKS before and after surgery. Responsiveness was compared using the standardized response mean (SRM). Stratified analyses based on change scores in the OKS were utilized to investigate how changes in the outcome measures related to each other. Patients were grouped based on the preoperative OKS to examine the relationship of change in the EQ-5D index and EQ-VAS with respect to initial health status.
RESULTS: For the overall cohort (54,486 patients), mean change scores pre/post knee replacement were 0.30 for the EQ-5D index (SD 0.33; SRM = 0.90), 3.3 for the EQ-VAS (SD 21.0; SRM = 0.16), and 14.9 for the OKS (SD 9.9; SRM = 1.50). The OKS changed uniformly with the EQ-5D index, but less concordantly with the EQ-VAS in response to knee replacement surgery. Substantial functional improvement was needed before mean EQ-VAS change scores showed improvement. Patients with worse preoperative health status had greater improvement following surgery, but the improvement in the EQ-5D index did not necessarily translate into comparable improvement in self-perceived well-being measured by the EQ-VAS.
CONCLUSIONS: On average, patients self-rated their health systematically lower using the EQ-VAS compared to the EQ-5D index and OKS following knee replacement. The EQ-VAS captured information about how patients feel about their health pre-/post-surgical intervention that contrasted with more functional measures of health. Additional qualitative research is needed to better understand these differences.