QALYs: the basics.

The aim of this article is to review the concept of the qualityadjusted life-year (QALY), a widely used measure of health improvement that is used to guide health-care resource allocation decisions. The QALY was originally developed as a measure of health effectiveness for cost-effectiveness analysis, a method intended to aid decision-makers charged with allocating scarce resources across competing health-care programs [1–3]. We refer to this original concept of the QALY, as defined in the early literature, as the “conventional” QALY, recognizing that alternative conceptual models have been proposed, including but not limited to so-called “equity-weighted” QALYs. The US Panel on Cost-Effectiveness in Health and Medicine [4] and the National Institute of Health and Clinical Excellence (NICE) in Britain have both endorsed the conventional QALY for their “reference case,” i.e., a standardized methodological approach to promote comparability in cost-effectiveness analyses of different health-care interventions. In using QALYs, we assume that a major objective of decisionmakers is to maximize health or health improvement across the population subject to resource constraints. The use of QALYs further assumes that health or health improvement can be measured or valued based on amounts of time spent in various health states. The conventional QALY is therefore a valuation of health benefit. We note, however, that decision-makers may also have other objectives such as equity, fairness, and political goals, all of which currently must be handled outside the conventional Special Issue [5] addresses some of these variations on the conventional QALY. The QALY was not initially developed to aid individual patient decision-making, although its use has sometimes been extended into clinical decision analyses for this purpose. The core concept of the conventional QALY is grounded in decision science and expected utility theory. The basic construct is that individuals move through health states over time and that each health state has a value attached to it. Health, which is what we are seeking to maximize, is defined as the value-weighted time—life-years weighted by their quality—accumulated over the relevant time horizon to yield QALYs. Health states must be valued on a scale where the value of being dead must be 0, because the absence of life is considered to be worth 0 QALYs. By convention, the upper end of the scale is defined as perfect health, with a value of 1. To permit aggregation of QALY changes, the value scale should have interval scale properties such that, for example, a gain from 0.2 to 0.4 is equally valuable as a gain from 0.6 to 0.8. States worse than dead can exist and they would have a negative value and subtract from the number of QALYs. These conditions, along with an assumption of risk neutrality over life-years, are sufficient to ensure that the QALY is a useful representation of health state preferences.