DALY and QALY
By 2020, the burden of (communicable/noncommunicable) diseases is expected to increase.
Disability-adjusted life years or DALYs are a measurement of disease burden, which is the impact of a health problem on an individual or in a population.
Typically, disability-adjusted life years are used to compare the burden of two diseases, like type II diabetes - which is a chronic condition where the body doesn’t produce enough insulin - and malaria - which is an infectious disease spread by mosquitoes that causes a person to have flu-like symptoms.
These two diseases are usually hard to compare because they affect different populations, have different short-term and long-term complications, and are managed completely differently.
So, to compare the two diseases, you could calculate the disability-adjusted life years for both diabetes and malaria, and the disease with the highest disability-adjusted life years has the highest disease burden.
Disability-adjusted life years are particularly useful for helping determine how resources should be allocated to a specific health issue.
So let’s try to calculate the disability-adjusted life years for diabetes in Canada, and that requires knowing two things.
First, you need to know the years of life lost to premature death, or YLL.
The years of life lost is calculated by multiplying the number of deaths that were the result of the disease (N), and the standard life expectancy at the age of death (L).
The standard life expectancy is just the average life expectancy in a population minus the age of the person who died.
For example, the average life expectancy in Canada is 82 years old, so the standard life expectancy of a person at age 60 is 22 years, because 82 minus 60 is 22. And if we’re only talking about 1 person who died, the number of deaths is 1.
So the years of life lost for a 60-year-old person that died from diabetes would be 1 times 22, or 22 years.
Calculating the years of life lost for an entire population gets a little more complicated, since a population is usually made up of people in different age groups.
For example, let’s say you want to figure out the burden of diabetes in a population of 4,000 people randomly selected in Canada.
So, you recruit 1,000 individuals in each of the 4 different age categories - 0 to 19, 20 to 49, 50 to 79, and above 80 - and follow them for 10 years.
You find out that there was 1 death from diabetes in the youngest age group, then 3, 4, and 7 deaths in the older age groups.
You also find that the average age of death for each age group is 19, 40, 65, and 81, so you’re able to calculate the average life expectancy at the age of death using the average life expectancy in Canada - which is 82. So 82 minus 19 is 63; 82 minus 40 is 42; 82 minus 65 is 17; and 82 minus 81 is 1.
Finally, you can calculate the years of life lost in each age group by multiplying the number of deaths in each age group by the average life expectancy for each age group. So, 1 times 63 is 63; 3 times 42 is 126; 4 times 17 is 68; and 7 times 1 is 7.
When you add up all of the years of life lost in each age group, the total number of years of life lost is 264.
The second thing you need to calculate the disability-adjusted life years is the number of years lost due to disability, which is sometimes referred to as the years lived with disability, or YLD.
The years lost due to disability is calculated by multiplying the incidence rate of the disease - which is the number of new cases of the disease that occurred within a certain time period - the length or duration of disease - which is the average length of time a person had the disease before dying, and the disability weight - which is a number assigned to each disease that represents the severity of the disease, or how much a disease affects how a person lives their life.
Disability weights range between 0 and 1, and in general, diseases with a score of 0 don’t limit a person’s ability to participate in any recreational, educational, procreational, or occupational activities.
People with diseases that have disability weights of 0.2 to 0.6 tend to have limited ability in some or all of those activities, and people with diseases that have disability scores of 0.8 or above typically need assistance with some or most daily activities.
For example, let’s say the disability weight for diabetes is 0.5. To calculate the years lost due to disability for a person in Canada who was diagnosed with diabetes at age 50, lived with diabetes for 10 years, and died at age 60, you would multiply the incidence - which would just be 1 in this case - times the duration - or 10 years - times the disability weight - or 0.5, which equals 5 years.
To calculate the number of years lost due to disability in a population you have to add up each of the years lost due to disability for each age group.
For example, let’s say that in 0-19 age group, there were 2 new cases of diabetes in the ten years of follow-up, so the incidence rate is 2 cases per 10 years for the youngest age group, and 9, 30, and 18 for the other age groups.
To figure out the average duration, you’d have to add up the number of years each person had diabetes until they died or until they moved into another age group.