In the first part of this series, we explored how quality of life is calculated, a complex process used by healthcare researchers as they attempt to prioritize time, energy and funds, with particular emphasis on individual cases. In today's post, we will look at a far less complex measure, Disability Adjusted Life-Years, or DALYs. This measure is used by epidemiologists, health policy-makers, pharmaceutical companies as well as the healthcare industry to inform decisions that affect whole populations of people.

DALYs | Years of healthy life lost | Scored by medical experts

In contrast to the Quality-Adjusted Life-Year (QALY) use of huge individual surveys to gauge a person's perception of quality of life, DALYs use expert medical opinion to quantify the burden of disease in terms of number of potential years of healthy life lost due to premature death. DALYs are calculated based on the severity of the disease, with each condition given a numeric value determined by medical experts.

DALYs allow researchers to compare the disease burdens of two or more conditions on a population. A policy-maker or healthcare system may consider the impact of heart disease and diabetes (two leading chronic conditions) on a population when deciding where to invest development funds. If heart disease DALYs show reveal more years of healthy life lost compared to diabetes, the decision-maker may choose to invest in preventing heart disease rather than diabetes.

DALYs are useful when looking at a population rather than an individual, as each patient will vary in their response to a disease and an intervention. Because of this big-picture approach, DALYs are very limited in their ability to examine disease burden by characteristics such as age, gender, socioeconomic status, race, pre-existing conditions, geographic location, and other social determinants of health. The Global Burden of Disease provides tables and rationale for over 350 conditions, allowing researchers to use standard metrics as a part of their data analysis.

As with any metric attempting to quantify an aspect of the human experiences, QALYs and DALYs have their weaknesses. In the case of QALYs, the quality of life for a given condition tends to be lower according to a person who has never experienced the disease or disability (those who have experienced it and learned to cope tend to rate the quality of life higher). DALYs, because they do not consider the variation in burden over the course of a disease, may not provide the necessary picture of the burden of living with a disease, both for the patient and society. Additionally, DALYs do not consider how a specific disability might limit a patient differently depending on social, economic, or built environments (a person without use of their legs will be more limited in an environment with low resources compared to a person with substantial resources). Despite their weaknesses, however, these two measures do provide needed quantification that, if used correctly and without losing sight of limitation, can be helpful in making healthcare decisions.

In our concluding post in this series, we will see how both these metrics are used in the field of infection control and prevention.