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Reliability, validity, and minimally important differences of the SF-6D in systemic sclerosis

Abstract

ObjectivesTo evaluate the reliability and validity and estimate the minimally important difference (MID) for the SF-6D in patients with systemic sclerosis (SSc). Subjects We 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 We 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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Acknowledgments

Dr. Khanna is supported by the Scleroderma Foundation (New Investigator Award), and a National Institutes of Health Building Interdisciplinary Research careers in Women’s Health (BIRCWH) Award (grant#HD051953). Drs. Furst, Wong, and Postlethwaite are partially supported by the Scleroderma Foundation (Established Investigator Award). Dr. Tsevat is supported in part by a National Center for Complementary and Alternative Medicine award (grant # K24 AT001676). Dr. Hays is supported in part by a grant from the National Institute of Aging (P01-AG-02-079), the UCLA/DREW Project EXPORT, National Institutes of Health, National Center on Minority Health & Health Disparities (P20-MD00148-01) and the UCLA Center for Health Improvement in Minority Elders/Resource Centers for Minority Aging Research, National Institute of Aging (AG-02-004). The Oral Type 1 Collagen in Scleroderma Study was sponsored by the National Institutes of Health (NIH)/National Institute of Arthritis and Musculoskeletal and Skin Diseases (grant #N01AR092242).

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Correspondence to Dinesh Khanna.

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On behalf of the Oral Type 1 Collagen in Scleroderma Study Group, the following investigators also participated:

University of Tennessee, Memphis: Andrew B. Kang, MD;

Boston University, Boston, Massachusetts: Robert Simms, MD; Joseph Korn, MD; Peter Merkel, MD, MPH;

Medical University of South Carolina, Charleston, South Carolina: Edwin Smith, MD;

Johns Hopkins School of Medicine, Baltimore, Maryland: Fred Wigley, MD; Barbara White, MD;

Georgetown University, Washington, DC: Virginia Steen, MD;

University of Texas Houston, Houston, Texas: Maureen Mayes, MD, MPH;

University of Alabama, Birmingham, Alabama: Larry Moreland, MD; Barri Fessler, MD;

Virginia Mason Research Center, Seattle, Washington: Jerry Molitor, MD, PhD;

University of Connecticut Health Center, Farmington, Connecticut: Naomi Rothfield, MD;

Hospital for Special Surgery, New York City, New York: Robert Spiera, MD.

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Khanna, D., Furst, D.E., Wong, W.K. et al. Reliability, validity, and minimally important differences of the SF-6D in systemic sclerosis. Qual Life Res 16, 1083–1092 (2007). https://doi.org/10.1007/s11136-007-9207-3

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Keywords

  • Systemic sclerosis
  • Scleroderma
  • SF-6D
  • Utility measures
  • Preference-based measures
  • Minimally important difference