DISABILITY ADJUSTED LIFE YEARS (DALY) :
Full economic evaluations are used to improve allocative efficiency in a healthcare system with limited resources by prioritizing among different interventions. However, prioritizing among different disease areas is not possible with economic evaluations. BoD studies are suitable for identifying unmet needs in healthcare. QALYs are suitable for estimating health capital, however they are not practical for estimating health loss in global burden of disease studies due to the high amount of input data needed. Therefore the WHO and the World Bank apply the DALYs to estimate health loss in global burden of disease studies.
The DALY has two key components :
DALY= Years of life loss (YLL) + Years lived with disability (YLD)
The impact of a particular disease on mortality is estimated by calculating the difference between life expectancy and the age at which death occurred and is expressed as years of life lost (YLL). To facilitate comparability across countries, the method is standardized by using the average life expectancy of Japanese women (82.5 yrs.) for women and an arbitrary value of 80 years is used for men 5. Therefore
YLL = average life expectancy – age at death
Years lived with disability (YLD) measures the impact of morbidity in the DALY concept, considering the following factors:
· The extent of disability associated with non-fatal conditions (disability weights), which applies an endpoint of 0.0 for perfect health and 1.0 for death.
· The relative importance of healthy life at different ages which are weighted according to productivity (age weights, which are the highest in the middle-aged group, and lower in the elderly and the young).
· The time preference for health (the value of health gained now as compared to the value of health gained in the future) (per protocol discounting with 3%).
Disability weights are determined by experts by using person-trade-off method (PTO).
DALYs can be used in burden of disease studies (WHO GBoD) and cost effectiveness studies (cost/avoided DALYs) to support evidence-based health policy. However, the applicability of DALY has been heavily criticized, especially when used for prioritization among health technologies.