The most widely used approach for estimating quality of life benefits in economic evaluations is the quality-adjusted life-year (QALY). In this approach, states of health are assigned a health state preference or 'utility' value, on a scale including 1.0 (full health) and 0 (death). The amount of time an individual spends in a given health state is then multiplied by the health state preference value to calculate the quality-adjusted life-years (QALYs) gained.
The QALYs gained from a given health care intervention are estimated by considering the difference in progression, through the various health states, with and without the intervention concerned. This is shown schematically in Figure 5.1. Here the intervention leads to QALY gains both by increasing or maintaining quality of life and by extending life. The main advantage of the QALY approach is that it provides one combined measure of the benefits of a program that both extends life and maintains quality of life.
In the context of HIV or HCV, we might expect that the health state values for early stages of disease, such as early HIV (CD4 count above 500/mm3) or mild chronic hepatitis, are higher than those for the later stages of disease, such as AIDS or liver failure. Therefore, if the NSP reduces the probability of infection, or increases the average time to infection, we would expect fewer individuals in a cohort of IDUs to progress to the later stages of disease during their lifetime. Under the QALY approach this will lead to QALY gains.
Figure 5.1 Quality-adjusted life years
Text version of Figure 5.1The diagram shows that without the program, as the quantity of life (years) increase the quality of life (weights) decreases from 1.0 to 0.0 (death). The diagram also shows that with the program, there is a gain in life years.
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