Measuring burden of disease Introduction to DALYs FETP India.
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Transcript of Measuring burden of disease Introduction to DALYs FETP India.
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