The Discriminative Properties of the SF-6D Compared With the SF-36 and ODI

Study Design. Longitudinal cohort. Objectives. To determine the discriminate validity of the Short Form-6D (SF-6D) compared with the SF-36 in a cohort of patients with lumbar degenerative disorders. Summary of Background Data. Cost-utility studies are important for the demonstration of comparative effectiveness of treatments for lumbar degenerative disorders. Multidimensional patient-reported outcome tools including the SF-36 and Oswestry Disability Index (ODI) may be limited to measuring the utility of specific health care states. The evaluation of utility is based on single-index preference-based health state scales, such as the SF-6D. The loss of discriminative properties using a single-index compared with a multidimensional score is unknown. Methods. The cohort studies included 1104 patients who had decompression and lumbar fusion with complete ODI, SF-36, and SF-6D data at baseline and 2-year follow-up. Discriminative properties of the 3 measures were compared by computing the effect size (ES) and the standardized response mean (SRM). The larger the ES and SRM, the more sensitive to change the measure is. The relative validity (RV) statistic for each measure was also determined with the SF-6D as reference. Measures that are more sensitive than the SF-6D would have RVs greater than 1.0, those that are less sensitive would have RVs less than 1.0. Results. The ODI had the greatest ES at 0.93 followed by the SF-6D at 0.88 and the SF-36 Physical Composite Score (PCS) at 0.85. The ODI also had the greatest standardized response mean at 0.73 followed by the SF-6D at 0.70 and the SF-36 PCS at 0.57. The RV statistics for both the ODI (1.28) and the SF-36 PCS (1.32) were greater compared with the SF-6D. The SF-36 bodily pain domain had the greatest ES (1.42), SRM (0.81), and RV (1.50). The general health domain had the lowest ES (0.21) and SRM (0.23), whereas mental health domain had the lowest RV. Conclusions. In this cohort of patients, using the single-index SF-6D produces a loss of discriminative properties compared with the SF-36 and ODI. However, this loss is small, because all the effect sizes remain large (>= 0.80). Therefore, these losses should not preclude the use of the SF-6D compared with the SF-36 and may be preferred because it is more easily interpretable and less difficult to incorporate in economic evaluations.