Reliability, Validity, and Minimally Important Differences of the SF-6D in Systemic Sclerosis

Published in: Quality of Life Research, v. 16, no. 6, Aug. 2007, p. 1083-1092

Posted on RAND.org on January 01, 2007

by Dinesh Khanna, Daniel E. Furst, Weng Kee Wong, Joel Tsevat, Philip J. Clements, Grace S. Park, Arnold E. Postlethwaite, Mansoor Ahmed, Shaari Ginsburg, Ron D. Hays

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OBJECTIVES: To evaluate the reliability and validity and estimate the minimally important difference (MID) for the SF-6D in patients with systemic sclerosis (SSc). SUBJECTS: The authors used data from two clinical studies to analyze the SF-6D in patients with SSc: Study 1 was a cross-sectional observational study (N = 107) designed to assess three direct preference measures--the rating scale, time trade-off, and standard gamble (SG) in patients with diffuse SSc and limited SSc, and Study 2 was a 12-month randomized, placebo-controlled, clinical trial (N = 168) assessing oral bovine collagen versus placebo in diffuse SSc. METHODS: The authors assessed the test-retest reliability of the SF-6D in Study 2 over a mean (SD) 4.8 (3.0)-week interval and the agreement between the SF-6D and direct preference measures in Study 1 using intraclass correlations (ICC). The MID was estimated using three different anchors--the SF-36 change in health item (patients who answered somewhat better formed the MID group), the Health Assessment Questionnaire-Disability Index (HAQ-DI; change of =0.14 and =0.22) and the skin score (change of =5.3). RESULTS: The mean (SD) SF-6D scores were 0.61 (0.12) in Study 1 and 0.64 (0.13) in Study 2. Test-retest reliability for the SF-6D was high (ICC = 0.82 [95% CI: 0.76, 0.87]). Agreement between the SF-6D and three direct preferences measures was poor to moderate (0.16-0.52). The MID estimate for the SF-6D using the change in SF-36 item -0.012 and this level of change was similar to the no change group. The mean MID estimate for the SF-6D improvement using the HAQ-DI and skin score as anchors was 0.035 (effect size of 0.27). CONCLUSION: This is the first study to assess the SF-6D in SSc. The SF-6D is reliable and valid in patients with SSc. We provide MID estimates that can aid in calculating sample size for clinical trials involving patients with diffuse SSc.

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