Utilities derived from visual analog scale scores in patients with HIV/AIDS.
BACKGROUND
Visual analog scale (VAS) scores are used as global quality-of-life indicators and, unlike true utilities (which assess the desirability of health states v. an external metric), are often collected in HIV-related clinical trials. The purpose of this study was to derive and evaluate transformations relating aggregate VAS scores to utilities for current health in patients with HIV/AIDS.


METHODS
HIV-specific transformations were developed using linear and nonlinear regression to attain models that best fit mean VAS and standard gamble (SG) utility values directly derived from 299 patients with HIV/AIDS participating in a multicenter study of health values. The authors evaluated the transformations using VAS and SG utility values derived directly from patients in other HIV/AIDS studies. Derived transformations were also compared with published transformations.


RESULTS
A simple linear transformation was derived (u = 0.44v + 0.49), as was the exponent for a curvilinear model (u = 1 - [1 - v]1.6), where u = the sample mean utility and v the sample mean VAS score. The curvilinear transformation predicted values within 0.10 of the actual SG utility in 5 of 8 estimates and within 0.05 in 3 of 8 estimates (absolute error ranged from -0.01 to +0.21). The linear transformation performed somewhat better, predicting within 0.10 of the actual SG value in 6 of 8 cases and within 0.05 in 5 of 8 estimates (absolute error ranged from -0.05 to +0.13). An alternative linear model (u = v + 0.018) derived from the literature performed similarly to our linear model (7 of 8 predictions within 0.10, 1 of 8 estimates within 0.05, and absolute error ranging from -0.15 to +0.10), whereas an alternative published curvilinear model (u = 1 - [1 - v]2.3) performed the least well (2 of 8 estimates within 0.10 of the actual values and no estimates within 0.05).


CONCLUSIONS
Predicted utilities are a reasonable alternative for use in HIV/AIDS decision analyses and cost-effectiveness analyses. Linear transformations performed better than curvilinear transformations in this context and can be used to convert aggregate VAS scores to aggregate SG values in large HIV/AIDS studies that collect VAS data but not utilities.
Mrus
Joseph M
JM
Health Services Research and Development, Cincinnati VA Medical Center, Ohio, USA. joseph.mrus@uc.edu
Yi
Michael S
MS
Freedberg
Kenneth A
KA
Wu
Albert W
AW
Zackin
Robert
R
Gorski
Heather
H
Tsevat
Joel
J
D000163:Acquired Immunodeficiency Syndrome / Q000503:physiopathology / Q000523:psychology*; D015331:Cohort Studies; D003627:Data Interpretation, Statistical; D015658:HIV Infections / Q000503:physiopathology / Q000523:psychology*; D006304:Health Status; D006305:Health Status Indicators; D006801:Humans; D016014:Linear Models; D010147:Pain Measurement / Q000706:statistics & numerical data*; D012720:Severity of Illness Index; D014481:United States