Relationship between quality of life instruments, health state utilities, and willingness to pay in patients with asthma.

BACKGROUND: In asthma, a substantial impact of disease is on quality of life rather than survival. To date, the quality of life effects of asthma have not been quantitated.
OBJECTIVE: The purpose of this study was to quantitate the quality of life effects of asthma via measurement of the health utility of asthmatic patients, and the willingness to pay for an asthma cure. A second goal was to analyze how these measures are related to the different dimensions of quality of life as measured by general and disease specific quality of life instruments.
METHODS: Health utilities were measured on a scale between 0 and 1 using the rating scale, time trade off, and standard gamble methods. Willingness to pay was elicited using both the dichotomous choice and the bidding game approach. Quality of life was assessed using both a generic instrument (the SF-36) and a disease-specific instrument (the Asthma TyPE).
RESULTS: Sixty-nine patients with asthma were surveyed. The mean health utility was 0.68 with the rating scale method, 0.89 with the time trade off, and 0.91 with the standard gamble. On average, patients were willing to pay between $200 to $350 dollars more per month for an asthma cure. Nearly all correlations between dimensions of quality of life, health state utilities, and willingness to pay were in the expected direction.
CONCLUSION: The two quality of life instruments performed about equally well in terms of being correlated with and being able to explain the responses to the health state utility questions and the willingness to pay questions. Correlations between the dimensions of the Asthma TyPE and the SF-36 were also fairly high. Further work should focus on validating and refining the different methods of quantitating quality of life for asthma patients.