Predicting utility ratings for joint health states from single health states in prostate cancer: Empirical testing of 3 alternative theories

Background. Cost-effectiveness analyses measure quality of life by associating utilities with specific health states. Utililies are often defined by single health states, such as incontinence or impotence in the case of prostate cancer treatments. Health conditions often occur simultaneously, yielding joint health states (e.g., impotence with incontinence). Given the combinatorial mathematics involved, even a small number of conditions can result in large numbers of potential joint states, complicating utility elicitation for all relevant states. Analytic predictions for joint-state utilities have been based on 3 theoretical models: 1) multiplicative, 2) additive, and 3) minimum models. These models' empirical accuracy for joint-state utility prediction has been minimally examined. The authors compared these 3 models for predicting joint-state utilities from single-state utilities in men at the time of prostate biopsies. Methods. Utilities were collected using time tradeoff in 2 university-based prostate biopsy clinics (N = 147). Single-state utilities were elicited for impotence, incontinence, watchful waiting, and postprostatectomy. joint-state utilities were elicited for states combining impotence with 1) incontinence, 2) postprostatectomy, or 3) watchful waiting. Testing 3 prediction models of joint-state utilities for bias and consistency, the predictions were compared against directly elicited joint-state utilities. Results. All 3 models are biased. The minimum model is preferred, being the least biased and most efficient, Conclusions. No current model accurately predicts joint-state utility using the component single-state utilities. When possible, joint-state utilities should be elicited. if not possible, the minimum model is recommended. Research to identify better models is needed.