Separating gains and losses in health when calculating the minimum important difference for mapped utility measures

Objective To estimate the minimum important difference (MID) for a variety of mapped utility measures and to determine whether patients perceiving gains and losses in health status should be treated equally when calculating the MID. Methods A longitudinal study within a California managed care population of 6,932 patients was retrospectively analyzed. Utilities were derived from the SF-36 short-form health survey using multiple validated mapping methods. Absolute utility changes for patients who considered their current health as 'somewhat better' or 'somewhat worse' in the prior year were compared to determine if gains and losses in utility values could be combined. The MIDs were calculated and compared using anchor- and distribution-based methods. Results Two thousand one hundred patients reported 'somewhat better' or 'somewhat worse' health in the first year. When combining these patients, the average MID for all mapped utility measures was 0.03 (SD = 0.1), a magnitude similar to that identified by Walters. However, when separated, the mean MID utility change for those reporting 'somewhat better' and 'somewhat worse' health was 0.02 (SD = 0.1) and -0.06 (SD = 0.1), respectively (P < 0.0001). Conclusions Researchers should consider the effects of combining gains and losses when determining utility MID values.