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

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 Birth Life expectancy Early death Birth Life 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 YLLs Life 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 0Year 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 1Year 2Year 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 1Year 2Year 3 4 years lived 100%: No DALY lost 4 years lived 75% (Disability weight [DW]: 25%): One DALY lost2 years lived 100%, 2 years lived 50% (DW: 50%): One DALY lost3 years lived 100%, 1 year lived 0%: One DALY lost Total: 3 DALY lost Life expectancy DALYs

Year 0 Year 1Year 2Year 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 YLLs DALYs

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 0Year 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 CasesDeathsYLLsDALYs Allow comparisons for the same disease Allow comparisons between diseases Take age at death into account Take disability into account