Quality-Adjusted Life Year (QALY)

The quality-adjusted life year (QALY) is a measure of health outcome that combines length of life and quality of life into a single number. One year of life in full health counts as one QALY; a year lived in a worse health state counts as a fraction, weighted by a utility value between 0 (a state regarded as equivalent to death) and 1 (full health). By placing survival and quality of life on a common scale, the QALY lets very different interventions be compared in cost-utility analysis.

Definition

A quality-adjusted life year is a unit of health outcome equal to one year of life in full health, calculated by multiplying time spent in a health state by a utility weight (between 0 and 1) representing the quality of that state.

Scope

This entry covers what the QALY is, how health-state utilities are elicited and used to weight time, the QALY's role as the effect measure in cost-utility analysis, and the principal criticisms of the measure. It is a methodological reference within HTA and health economics and does not set thresholds or treatment recommendations.

Core questions

• How is the utility weight for a health state determined?
• How are time and utility combined to produce QALYs?
• How are QALYs used as the effect measure in cost-utility analysis?
• What assumptions and value judgements does the QALY embed?

Key concepts

• Health-state utility (0 to 1 scale)
• Utility elicitation (standard gamble, time trade-off, rating scale)
• Multi-attribute utility instruments (e.g. EQ-5D)
• Cost-utility analysis and cost per QALY
• Willingness-to-pay threshold per QALY
• Equity and distributional criticisms of the QALY

Mechanisms

Constructing a QALY requires a utility weight for each relevant health state. Utilities are elicited using preference-based methods — the standard gamble, the time trade-off, and rating scales — or read from multi-attribute instruments that map a descriptive health profile onto a utility value; Torrance (1986) set out the measurement basis for these approaches. Time spent in each state is multiplied by its utility and summed (often with discounting) to give total QALYs, so that gains from extending life and from improving quality of life are captured together. Weinstein and Stason (1977) embedded this outcome measure within cost-effectiveness analysis, where the cost per QALY gained (the ICER of a cost-utility analysis) becomes the basis for comparing interventions, sometimes against a willingness-to-pay threshold (Shiroiwa et al., 2010). Whitehead and Ali (2010) review how QALYs and utilities function in economic evaluation and the debates surrounding them.

Clinical relevance

The QALY is the common currency by which cost-utility analyses compare the value of diverse interventions, and it underlies many coverage decisions. It is a reference outcome measure used in population-level appraisal and is not a tool for individual clinical decision-making.

Evidence & guidelines

Standard methods for measuring utilities and applying QALYs are described by Torrance (1986), Whitehead and Ali (2010), and economic-evaluation texts such as Drummond et al. (2015); the use of QALYs within cost-effectiveness analysis traces to Weinstein and Stason (1977). Empirical work on willingness-to-pay thresholds per QALY (Shiroiwa et al., 2010) informs how cost-per-QALY results are interpreted.

History

The idea of weighting life-years by quality emerged in health economics in the 1960s and 1970s, and the QALY was consolidated as cost-effectiveness analysis took shape; Weinstein and Stason's 1977 paper integrated quality-adjusted outcomes into the analytic framework. Torrance's work through the 1970s and 1980s formalised the measurement of health-state utilities, and the QALY became the standard effect measure for cost-utility analysis, later operationalised through generic instruments such as the EQ-5D.

Debates

Does the QALY fairly value health gains across groups?

Critics argue the QALY can disadvantage older people and those with disabilities or chronic conditions, because a given absolute improvement may translate into fewer QALYs; how to weigh efficiency against equity in QALY-based decisions remains contested.

Is there a defensible cost-per-QALY threshold?

Whether and at what level a willingness-to-pay threshold per QALY should be set is debated, with empirical surveys finding wide variation across countries and contexts.

Key figures

• George W. Torrance
• Milton C. Weinstein
• William B. Stason
• Sarah J. Whitehead

Seminal works

• weinstein-stason-1977
• torrance-1986
• whitehead-ali-2010

Frequently asked questions

What does one QALY represent?

One QALY represents one year of life lived in full health. A year in a less-than-perfect health state counts as a fraction of a QALY, weighted by a utility value between 0 and 1.

How is the quality weight for a health state obtained?

It is elicited using preference-based methods such as the standard gamble or time trade-off, or derived from multi-attribute utility instruments that map a described health state onto a utility value.