The purpose of the current study was to apply a single large longitudinal EQ-5D-3L data set to several national EQ-5D-3L value sets and explore differences in EQ-5D-3L index density functions and effect sizes before and after treatment.
Patients, surgically treated for lumbar spinal stenosis or lumbar disk herniation between 2007 and 2017, were recruited from the national Swedish spine register. A total of 27,328 procedures were eligible for analysis. The EQ-5D health states were coded to EQ-5D-3L summary indices using value sets for 9 countries: Argentina, Australia, Canada, China, Germany, Italy, Sweden, the UK, and the US. The EQ-5D-3L summary index distributions were then estimated with kernel density estimation. The change in EQ-5D-3L index before and after treatment was evaluated with the standardized response mean (SRM).
There was a high variability in the resulting EQ-5D-3L index density functions. There were also considerable differences in EQ-5D-3L index density functions before and after treatment using the same value set. Effect sizes of 2-year change (SRM), however, were similar when the 9 value sets were applied on pre- and post-treatment data.
We found a marked variability in EQ-5D-3L index density functions when a single large data set was applied to 9 national EQ-5D-3L value sets. Consequently, studies that aggregate international data, e.g. meta-analyses, may produce misleading results if the underlying differences in EQ-5D-3L index density functions are inadequately handled. On the basis of the results of our study, we recommend against pooling of different national EQ-5D-3L index data.