Combining utility measurements - Exploring different approaches

Background: The use of utility values in cost-effectiveness analysis is an accepted method for defining outcomes. Increasingly, cost-effectiveness analyses examine outcomes that involve two or more health states. There is no accepted method of mathematically combining single health state,utility values into a surrogate value that represents the combined health state. Objective: To test the effect of different mathematical approaches to combining single health state utility values into a surrogate value on the cost-effectiveness ratio, in a sample model. Methods: We employed a realistic decision analysis model to test the cost-effectiveness of screening for postpartum thyroiditis. Utility values for type I diabetes mellitus and thyroiditis were taken from the literature and combined using different methods. Results: The surrogate utility values obtained using the multiplicative method were higher than those obtained with the additive method (for example, the state of both type I diabetes mellitus and treated thyroiditis had a value of 0.75 for the multiplicative method versus 0.73 for the additive method). The resulting cost-effectiveness ratios for the screening strategy were slightly higher, $US16 000 (1998 values) per quality-adjusted life-year (QALY), for the multiplicative method when compared to the additive method ($US14 000 per QALY). This small difference was consistently maintained during sensitivity analyses. Conclusion: All methods of combining utilities resulted in similar values. Until a consensus is reached on the method of choice, researchers should consider using both methods in sensitivity analyses and reporting both sets of results.