OBJECTIVES

A within-trial DPP cost-effectiveness analysis (CEA) was published in 2003. The authors reported QALYs using utilities from the QWB for lifestyle (LS), placebo (PBO), and metformin (MET). Available to them, but not used, were more complete SF-6D results (calculable from their SF-36 data). QALY calculations were also not done correctly in the DPP. The main objective of this study is to estimate utilities and incremental QALYs using both instruments and correct method of calculation, comparing them and suggesting implications for CEA.

METHODS

We calculated QALYs correctly for the QWB results and compared them to the original reporting. We then mapped the SF-36 data to the SF-6D and calculated QALYs as in the original DPP CEA (incorrectly) and in the correct manner. The corrected calculation used area under the curve, based on beginning and end of the year utilities. It required a baseline utilities values, not included in the original work. If in common across the treatments, the baseline drops out of the QALYs differences, but the corrected method still implies changes in these differences.

RESULTS

The incremental QALYs using the QWB as originally calculated were 0.072, 0.022 and 0.050 for LS versus PBO, MET versus PBO, and LS versus MET. Corrected incremental QALYs were 0.056,0.015 and 0.041. Using the SF-6D utilities, incremental QALYs using incorrect and correct calculations were 0.031,0.002,0.029 and 0.027,0.003,0.024.

CONCLUSIONS

QWB and SF-6D instruments in the DPP yielded different incremental QALYs in original and corrected calculations. Small differences in QALYs have been shown previously to be important in the DPP CEA conclusions. These differences are likely important in their CEA implications.