Disability-Adjusted Life-Years

The disability-adjusted life-year (DALY) is a summary measure of population health that adds together time lost to premature death and time lived in less than full health. One DALY equals one lost year of healthy life, which lets the burden of very different diseases be compared on a single scale. It is the central metric of the Global Burden of Disease enterprise.

Definition

A disability-adjusted life-year is the sum of years of life lost to premature mortality (YLL) and years lived with disability (YLD), where one DALY represents one lost year of full health.

Scope

The entry covers the construction of the DALY from its two components — years of life lost and years lived with disability — the role of disability weights, the value choices embedded in the metric (age-weighting and discounting), and how the DALY differs from the quality-adjusted life-year used in cost-utility analysis. It is a methodological reference rather than clinical guidance.

Core questions

• How are years of life lost and years lived with disability combined into a DALY?
• How are disability weights chosen and what do they represent?
• What value judgements (discounting, age-weighting) are built into the metric?
• How does the DALY differ from the QALY?

Key concepts

• Years of life lost (YLL)
• Years lived with disability (YLD)
• Disability weight
• Reference life expectancy
• Discounting of future health
• Age-weighting
• DALY versus QALY

Mechanisms

The DALY is built additively. Years of life lost weight each death by the years of life expectancy remaining at the age of death, measured against a standard reference life table. Years lived with disability weight time spent in a health state by a disability weight on a 0-to-1 scale, where 0 is full health and 1 is a state equivalent to death. Summing YLL and YLD over a population gives total DALYs. Early formulations also discounted future years and applied age-weights that valued years in young adulthood more; later GBD cycles removed both, leaving a simpler additive metric.

Clinical relevance

The DALY summarises how much healthy life a disease removes from a population and is used to compare conditions and track change over time; it also serves as the effectiveness denominator in some cost-effectiveness analyses (cost per DALY averted). It is a population and policy measure and does not inform the diagnosis or treatment of an individual.

Epidemiology

DALYs are the headline output of the Global Burden of Disease studies, which estimate them for hundreds of diseases across more than two hundred countries and consistently show non-communicable diseases accounting for a growing share of global DALYs as populations age.

Evidence & guidelines

The DALY's methods are documented across the GBD systematic analyses, and its relationship to the QALY is treated in the health-economics methods literature. Because disability weights and value choices affect results, transparent reporting of the version and assumptions used is expected when DALYs are presented.

History

The DALY was introduced with the original Global Burden of Disease Study in the early 1990s as a common currency for fatal and non-fatal health loss, and refined through Murray and Lopez's syntheses. The GBD 2010 DALY analysis (Murray and colleagues, 2012) recalculated the metric for 291 conditions and dropped the earlier age-weighting and discounting conventions.

Debates

How should disability weights be set?

Whether weights should come from expert panels or general-population surveys, and whose preferences they represent, affects how non-fatal conditions rank; this has been revised across GBD cycles.

Should the DALY discount or age-weight future health?

Original DALYs discounted future years and weighted years by age; critics argued these embed contested ethical judgements, and later GBD versions removed them, showing the metric is partly a value construct.

Key figures

• Christopher Murray
• Alan Lopez
• Theo Vos
• Franco Sassi

Seminal works

• murray-1997
• murray-2012-dalys

Frequently asked questions

What does one DALY mean?

One DALY equals one year of healthy life lost, combining time lost to early death and time lived with disability.

How is a DALY different from a QALY?

A DALY measures health lost and is summed across a population (lower is better), whereas a QALY measures health gained and is typically used in cost-utility analysis (higher is better); the two are related but built and used differently.