Measuring burden of disease

Introduction to DALYs

FETP India

Objective of this lecture

Understand methods used to estimate burden of disease in terms of

death and disability

Key areas

• Cases• Deaths • Years of life lost (YLLs)• Disability-adjusted life years (DALYs)• Age weighting • Discounting

Reasons to measure burden of disease

• Assign priorities between different public health problems

• Compare strategies that a single public health problem in terms of impact

• Choose health interventions that target different health problems with different strategies

Cases

Cases

• Allow comparing health interventions targeting the same problem

• Do not allow Comparisons between diseases

• Does not take into account Case fatality Disability

Cases

Cases: Examples

• Number of cases of malaria• Number of cases of tuberculosis• Number of HIV infections

Cases

Deaths

• Allow comparing health interventions targeting the same problem

• Allow comparing different diseases• Does not take into account

Age at death Disability

Deaths

Deaths: Examples

• Comparing the number of deaths from malaria with the number of deaths from tuberculosis

• Estimating the number of deaths prevented through a sanitation programme

Deaths

Years of life lost (YLLs)

• Allow comparing health interventions targeting the same problem

• Allow comparing different diseases• Take into account age at death• Do not take into account

Disability

YLLs

Years of life lost

BirthLife

expectancy

Early deathBirthLife

expectancy

Years of life lost

YLLs

• Information required Age at death Sex Country / region

• Estimating life expectancy Depends upon age, sex and region Obtained in Global Burden of Disease life

tables Years of life lost = Life expectancy - age at death

Years of life lost to present deaths (1/3)

YLLs

Life expectancy

Death today

Life expectancy

Years of life lost

Years of life lost to present deaths (2/3)

Survival today

YLLsLife expectancy at the age of death

• What is the number of years of life lost to AIDS in India in 2000?

• Elements needed: 2000 AIDS death rates (N) Age distribution of death cases (Age) Life expectancy (LE) at the age of death

Years of life lost to present deaths (3/3)

YLL = N x (LE-Age) Age groups

YLLs

• Year by year cohort analysis • Estimate mortality secondary to other

causes (years censored)

• Estimate number of deaths that will occur because of the disease studied Natural history

• Calculate years of life lost through subtracting age at death to life expectancy

Years of life lost to future deaths (1/3)

YLLs

Years of life lost to future deaths (2/3)

Year 0 Year 1

Years of life lost

Death from other causes

Year 2

Years of life lost

Death from other causes

Year 3

Years of life lost

Death from other causes

Life expectancy

YLLs

• What will be the number of years of life lost between 2000 and 2030 because of HIV infections acquired in India in 2000?

• Elements needed: HIV infections in India in 2000 Age distribution of cases of infection Background mortality by age AIDS specific death rate, year by year after

infection

Years of life lost to future deaths (3/3)

YLLs

Disability-adjusted life years (DALYs)

• Allow comparing health interventions targeting the same problem

• Allow comparing different diseases• Take into account age at death• Take into account disability

DALYs

The concept of disability

• Life in a state of perfect health is lived to its full potential

• Life in case of disease is lived lived partially Disability weight

DALYs

Measuring death and disability with the same metric

Year 0 Year 1 Year 2 Year 3

4 years lived 100%

4 years lived 75%

2 years lived 100%, 2 years lived 50%

3 years lived 100%, 1 year lived 0%

Life expectancy

DALYs

Disability-adjusted life years (DALYs)

• Summary measure• Take into account loss due to

Death (Years of life lost, YLLs) Disability (Years lived with disability, YLDs)

• The equivalence is: Death = 100% disability Perfect health = 0% disability

DALYs

The equivalence between disability and death

Year 0 Year 1 Year 2 Year 3

4 years lived 100%: No DALY lost

4 years lived 75% (Disability weight [DW]: 25%): One DALY lost

2 years lived 100%, 2 years lived 50% (DW: 50%): One DALY lost

3 years lived 100%, 1 year lived 0%: One DALY lostTotal: 3 DALY lost

Life expectancy

DALYs

Year 0 Year 1 Year 2 Year 3

1 year lived 100%, 2 years lived 50%, 1 year lost: 2 DALYs lost

Calculating DALYs

• Calculate years of life lost through death

• Calculate years of life lived with disability

• Add the two

DALYs

DALYs lost to present conditions (1/3)

• Estimate years lived with disability (YLDs): Estimate the prevalence of the disease

(e.g., viral hepatitis B) Estimate the prevalence of the sequalae of the disease

• Hepatocellular carcinoma (Specific disability weight)• Decompensated cirrhosis (Specific disability weight)• Chronic active hepatitis (Specific disability weight)

Multiply number affected by the disability weights

• Estimate years of life lost (YLLs) because of death: Specific mortality Calculate YLLs using life expectancy

• Add YLDs and YLLsDALYs

DALYs lost to present conditions (2/3)

Year of the study

Years of life lost

One year, 30% disability weight

One year, 50% disability weight

One year, 70% disability weight

Life expectancy

Total: 2.5 DALYs lost

DALYs

• Year by year cohort analysis • Background mortality to estimate mortality

secondary to other causes (years censored)• Estimate YLLs because of death

Natural history to estimate specific mortality Calculate YLLs using life expectancy

• Estimate YLDs Natural history to estimate morbidity and duration

secondary to the disease Multiply years at disability status by disability weight

• Add YLDs and YLLs

Calculating DALYs lost in the future

DALYs

Calculating DALYs lost in the future

Year 0 Year 1

Years of life lost

Death from other causes

Year 2

Years of life lost

Death from other causes

Year 3

Years of life lost

Death from other causes

Life expectancy

DALYs

The Global Burden of Disease Study

• Goes against disease-specific approaches • Integrates all causes of death and disability• Uses death certificates as starting point

The “mortality envelope” • Assigns deaths to

Immediate causes Risk factor to health

• Uses DALYs as the universal metric to compare public health issues

DALYs

Age weights

• One year lost does not have the same meaning according to the age

• Age weights attribute lower weights for Childhood Older age

Age weights

Discounting

• DALYs do not have the same value if lost Today (e.g., measles) Later (e.g., hepatitis B)

• Discounting rate Rate that discounts the value each year Apply to costs and effects

• E.g., 3% Subject to discussion

• Should DALYs be discounted?• At what rate?

Discounting

Comparison between various measures of burden

Cases Deaths YLLs DALYs

Allow comparisons for the same disease

Allow comparisons between diseases

Take age at death into account

Take disability into account