Reliability, validity, and minimally important differences of the SF-6D in systemic sclerosis.
OBJECTIVE
To 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 > or =0.14 and > or =0.22) and the skin score (change of > or =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).


CONCLUSIONS
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.
Khanna
Dinesh
D
Division of Immunology, Department of Internal Medicine, University of Cincinnati Medical Center, PO Box 670563, Cincinnati, OH 45267-0563, USA. dinesh.khanna@uc.edu
Furst
Daniel E
DE
Wong
Weng Kee
WK
Tsevat
Joel
J
Clements
Philip J
PJ
Park
Grace S
GS
Postlethwaite
Arnold E
AE
Ahmed
Mansoor
M
Ginsburg
Shaari
S
Hays
Ron D
RD
D000328:Adult; D000368:Aged; D003094:Collagen / Q000627:therapeutic use; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D017063:Outcome Assessment, Health Care / Q000379:methods*; D017060:Patient Satisfaction; D010919:Placebos; D011594:Psychometrics / Q000295:instrumentation*; D011788:Quality of Life; D012595:Scleroderma, Systemic / Q000188:drug therapy* / Q000503:physiopathology / Q000523:psychology*; D018434:Sickness Impact Profile; D012885:Skinfold Thickness; D014481:United States
D010919:Placebos; D003094:Collagen