DALY and QALY

Definitions & Key takeaways

The Disability-Adjusted Life Year (DALY) and Quality-Adjusted Life Year (QALY) are measures of population health used to quantify the burden of disease and injury. A DALY is a measure of lost healthy life years, while a QALY is a measure of how many years of life are lived in good health. Both measures incorporate the severity and duration of illness or injury and the age at which it occurs. They can be used to compare different diseases or injuries, or to compare the burden of disease between different populations.

Watch video only
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.
For example, if you have 100 thousand dollars to donate, do you want to spend it towards helping people with type II diabetes in Canada or helping people with malaria in India?
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.
For example, if a person was diagnosed with diabetes at age 73 and lived with the disease for 10 years, their duration in the 50-79 age group would be 6 years, and their duration in the 80 and above age group would be 4 years.
So let’s say the average duration for each of the age groups is 7, 16, 20, and 3 years. Finally, the disability weight for diabetes is 0.5 for each age group.
Now, to get the years lost due to disability for each age group, multiply the incidence rate, the average duration, and the disability weight for each age group, so 2 times 7 times 0.5 is 7; 9 times 16 times 0.5 is 72; 30 times 20 time 0.5 is 300; and 18 times 3 times 0.5 is 27.
To get the total years lost due to disability, add up all the years lost in each age group - so 7 plus 72 plus 300 plus 27, which equals 406 years lost due to disability.
The total number of disability-adjusted life years is the sum of the years of life lost to premature death and the years lost due to disability.
So, for people living in Canada, the years of life lost to premature death from diabetes was 264 and the years lost due to diabetes was 406.
So 264 plus 406 equals 670 disability-adjusted life years per 4,000 people. If we divide the number of disability-adjusted life years and the number of people by 4, we get 167.5 disability-adjusted life years per 1,000 people.
Now, we could compare that number to the disability-adjusted life years calculated for malaria in people living in India, and let’s say that number is 74 per 1,000 people.
Since 167.5 is greater than 74, you might decide to donate your 100,000 dollars to a group that’s helping people with diabetes in Canada.
Disability-adjusted life years are also used to compare the effect of a specific intervention, like a new medication that can help reduce the risk of diabetes.
Now, let’s say you donate the 100 thousand dollars to the development of this new medication, and after 5 years, you want to know if the new medication has reduced the burden of diabetes You find out that the disability-adjusted life years for diabetes is now 130.
Since you started with 167.5, that means 37.5 disability-adjusted life years were averted, or decreased, in 5 years, which means the new medication is very effective and that you made a good choice by donating to its development!
Typically, interventions that avert more disability-adjusted life years are considered better. Another measure used to quantify disease burden is called quality-adjusted life years, or QALYs.
Quality-adjusted life years are calculated by multiplying the number of years lived by a utility weight, which is basically the opposite of a disability weight.
Utility weights range from 0 to 1, but in this case 1 represents the best possible health and 0 represents death. Oftentimes, quality-adjusted life years are used to compare the cost of two interventions.
For example, let’s say you can choose to donate your money to the development of one new diabetes medication, and there are two options - Medication A and Medication B.
Now, let’s say Medication A has the potential to add 10 years to a person’s life and Medication B has the potential to add 6 years.
Also, Medication A costs 10,000 dollars per person while Medication B costs 15,000 dollars per person. At first, it seems like Medication A is the obvious choice.
But now let’s say the utility weight of Medication A is 0.2, while the utility weight of Medication B is 0.9, so a person taking Medication B will have a much higher quality of life compared to someone taking Medication A.
So, when you calculate the quality-adjusted life years for Medication A, 10 times 0.2, which equals 2, you find that it’s much lower than the quality-adjusted life years for Medication B, which is 6 times 0.9, or 5.4.
You can also calculate the cost of each medication per quality-adjusted life year by dividing the cost by the quality-adjusted life years.
So, for Medication A, that would be 10,000 divided by 2, which is 5,000 dollars per quality-adjusted life year; and for Medication B, that would be 15,000 divided by 5.4, which is around 2,778 dollars per quality-adjusted life year.
Comparing the two costs per quality-adjusted life year, you can see that donating your money to Medication B will give you the most bang for your buck!
Alright, as a quick recap, disability-adjusted life years or DALYs and quality-adjusted life years or QALYs are two ways to measure the burden of disease in a population.
Disability-adjusted life years can be calculated by adding the years of life lost to premature death with the years lost due to disability.
Years of life lost to premature death is calculated by multiplying the number of deaths that were the result of the disease and the standard life expectancy at the age of death.
And the years lost due to disability is calculated by multiplying the incidence rate of the disease, the duration of the disease, and the disability weight for that specific disease.
Quality-adjusted life years are calculated by multiplying the estimated number of years of survival by the utility weight, which is an estimation of the quality of life.
Interventions that avert the most disability-adjusted life years and create higher quality-adjusted life years are considered better.