What are Disability Adjusted Life Years or DALYs?
Thomas Bendall
All views my own | Product Strategy & Access Director | HTA, Reimbursement & HEOR | Strategic Planning | Access & Evidence | Pharmaceutical New Product Development & Positioning | Ambitious About Neurodiversity
Based on the WHO definition: One DALY represents the loss of the equivalent of one year of full health. They reflect years of premature life lost (YLL) and years lived with disability (YLD) due to the prevalence of disease in a population. In doing so DALYs provide a measure that combines mortality and morbidity.
DALYs as a measure were developed as efforts to quantify the burden of disease, as part of the Global Burden of Disease study - a study designed to provide a systematic and standardised effort to quantify the impact of all major diseases, allowing comparisons across populations, disease and time. There were three aims behind the development of the DALY: 1) to provide data on the relative magnitude of the impact of a disease or injury, which was not available when the GBD study was initiated; the intent was to provide informed estimates of disability that could then be used to support decision making; 2) provide a common way to evaluate non-fatal health outcomes driven by illness and disease, to enable and promote discussion, beyond mortality; 3) provide the means to quantify the impact of disease, enabling the assessment of allocative efficiency, using a single common measure.
DALYs can provide a population-level assessment. For this, DALYs are calculated via the addition of YLL and YLD (DALY = YLL + YLD). YLL is calculated by multiplying the number of deaths from a disease by the average life expectancy (in years), at the time of death. YLD is calculated by multiplying the incident number of cases by the disability weight by the duration of the condition.
The DALY weighting provides a 0-1 weighting, in a similar way to that seen with QALYs, with DALYs considered a variant of QALYs. DALYs and DALY weightings reflect health losses and as such the weighting scale is reversed, compared with QALYs, with 0 representing ‘good’ health and 1 representing ‘poor’ health, or most disability. Therefore in a simplistic form 1 DALY = 0 QALY & 0 DALY = 1 QALY.
The methodology used to generate disability weights has changed over the years. In the most recent methodology update, health states were generated with expert input and valuation was based on discreet choice experiments on vignettes of the health states, together with a population health equivalence question, to anchor ordinal data and to generate values on the 0-1 scale. In this update, a sample of over 30,00 respondents from a global, general population using postal, phone, and internet surveys.
Recommended by LinkedIn
From an evidence perspective, given the history and population-level focus of DALYs, DALYs tend to be used for assessments in low and lower-middle-income countries. They provide a simple to use, common outcome measure, where jurisdiction-specific QALY weights are not available. DALYs have also been incorporated into international decision-making criteria with organisations such as the World Health Organization, using DALYs as a common outcome measure for reporting, and the Bill & Melinda Gates Foundation including DALYs within the reference case for economic evaluation.