PURPOSE: Utility instruments measure quality of life by assessing the patient's preference for a given health state on a scale from 0 (corresponding to death) to 1 (perfect health). Multiplying utility by the years spent in a particular health state provides quality adjusted life years (QALY), a measure used in cost-effectiveness analysis. Until now it is uncertain which utility instruments are suitable for application in patients with obstructive sleep apnea syndrome (OSA). Therefore, we compared the effect of CPAP therapy in OSA patients on utility measured by various instruments.
METHODS: 65 patients with OSA were evaluated before and after 3 months of CPAP therapy. Utility was assessed by computerized standard gamble (SG) and time trade-off (TTO) tests, the SF-36 quality of life questionnaire-derived utility index (SF-6D), the EuroQuol questionnaire-derived utility index (EQ-5D), and the Euro-thermometer visual analog scale utility index (VAS). The Epworth sleepiness score and the apnea/hypopnea index were also obtained.
RESULTS: The median (quartiles) apnea/hypopnea index at baseline and at 3 months were 51 (38;74)/h and 6*(2;12)/h, corresponding Epworth scores were 12 (10;15) and 7*(5;10) (*P<0.00001). Baseline utility indices were: SG 0.97 (0.85;0.99); TTO 0.95 (0.81;0.99), EQ-5D 0.81 (0.76;0.92), VAS 0.80 (0.70;0.90), SF-6D 0.75 (0.68;0.85). After 3 months on CPAP, the changes in utility indices were: SG 0.00 (-0.02;0.08), TTO 0.00 (0.00;0.01), EQ-5D 0.00 (0.00;0.07), VAS 0.05* (0.00;0.15), SF-6D 0.04* (-0.01;0.11) (* P<0.01, change vs. baseline). Correlation among changes in utility assessed by different instruments was poor and only significant (P<0.01) for EQ-5D vs. VAS (Spearman R=0.37), and SG vs. TTO (Spearman R=0.41). There was no significant correlation among changes in utility indices and apnea/hypopnea index or Epworth score.
CONCLUSION: Utility indices measured with various instruments vary largely. Certain utility instruments do not reflect the impaired quality of life in untreated OSA patients and are unsuitable to track effects of CPAP therapy.
CLINICAL IMPLICATIONS: Cost-effectiveness analysis critically depends on the method used to evaluate utility. Therefore, the interpretation of cost-effectiveness analysis has to account for the utility instrument used.
DISCLOSURE: Konrad Bloch, No Financial Disclosure Information; No Product/Research Disclosure Information