Not all "quality-adjusted life years" are equal.

BACKGROUND There is evidence that utility elicitation methods used in the calculation of quality-adjusted life years (QALYs) yield different results. It is not clear how these differences impact economic evaluations. METHODS Using a mathematical model incorporating data on efficacy, costs, and utility values, we simulated the experiences of 100,000 hypothetical rheumatoid arthritis patients over 10 years (50,000 exposed to infliximab plus methotrexate [MTX] and 50,000 exposed to MTX alone). QALYs, were derived from the Health Utilities Index 2 and 3 (HUI2 and HUI3), the Short Form 6-D (SF-6D), and the Euroqol 5-D (EQ-5D). Incremental cost-utility ratios were determined using each instrument to calculate QALYs and the results were compared using cost-effectiveness acceptability curves. RESULTS Using the different utility measurement methods, the mean difference in QALYs between the infliximab plus MTX and MTX groups ranged from a high of 1.95 QALYs (95% CI=1.93-1.97) using the HUI3 to 0.89 QALYs (95% CI=0.88-0.91) using the SF-6D. Adopting the commonly cited value of society's willingness to pay for a QALY of $50,000, 91% of the simulations favored the cost utility of infliximab plus MTX when using the HUI3 to calculate QALYs. However, when using the EQ-5D, HUI2, or the SF-6D utility values to calculate QALYS, the proportion of simulations that favored the cost utility of infliximab were 63%, 45%, and 12%, respectively. CONCLUSION Depending on the method for determining utility values used in the calculation of QALYs, very different incremental cost-utility ratios are generated.