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Disability-Adjusted Life Years (DALYs)   Return to Disability Theme page

1. Core Knowledge:

What are these? DALYs offer a way to measure the combined quantity and quality of life of a population. They are often used to record the outcome of clinical interventions.

Why were they invented? Interventions that prolong life may be evaluated by recording the additional years of life lived by people who receive the intervention.  But this says nothing about the quality of those extra years: an intervention such as a transplant could extend a life but leave the patient in a disabled state, so each additional life-year gained is not really equivalent to a year of perfect health.  This introduces the concepts of disability-adjusted life years (DALYs) and of quality-adjusted life years (QALYs).   Both combine morbidity and mortality outcomes.

One DALY is equivalent to the loss of a healthy life year.  The computation counts the number of deaths attributed to a disease, but also incorporates the age at which the death occurs by counting the number of years the person would have been expected to live to. It then reduces this according to the level of disability of the person prior to death. 

These indicators can also be used to compare the impact of different diseases (or risk factors such as smoking) in terms of reducing quantity and/or quality of life, to set priorities for interventions.

2. Nice to Know

How are they calculated? Disability-adjusted life years (DALYs) focus on the negative impact of disease in modifying the picture given by longevity statistics. They record the length of life lost due to premature mortality (Years of Life Lost, or YLLs) and the years lived with disability (YLD).  Time lost to premature death is compared to a standard expectation of life (often 80 for men and 82 for women).  This is typically taken from life tables for the entire country, and the average life expectancy for people with the disease is compared to the overall life expectancy. Next, the part of a person's life lived with disability due to the condition is translated into an equivalent length of life lost, using a numerical judgment that represents the severity of disability.  For example, a man with a crippling disease that lasts 5 years might be judged to have lost the equivalent of 2 years of healthy life because of this disability.  If he contracts this condition at 69 and dies at age 74, he has lost 6 years of life expectancy (80 - 74), but in addition he has lost the equivalent of 2 further years due to the years of disability, to give a DALY of 8 years of healthy life lost due to this condition. 

Application: DALYs from different medical conditions can be calculated and compared for a population, as can the impact of various causal factors. The following chart illustrates the idea:

 

Ranking the impact of risk factors and diseases in terms of DALYs

The right column shows the impact of various disease categories (e.g., 9% of all DALYs lost are due to HIV/AIDS).  The left column shows the overall impact of risk factors, and arrow indicate the diseases through which they operate. The arithmetic is complicated because most of the diseases have multiple causes, so the impact of individual risk factors may be less than the sum of the DALY % for the diseases they cause. Note that both columns of numbers add to only around 50%, the remainder being due to a large number of very minor causes, so that multiple interventions will ultimately be required to have a substantial impact on population health.

Link to International comparison of DALYs