Transcript Getting to the essential

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
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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
Birth
Life
expectancy
Birth
Life
expectancy
Early death
Years of life lost
YLLs
Years of life lost to present deaths (1/3)
• 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
YLLs
Years of life lost to present deaths (2/3)
Survival
today
Life
expectancy
Death
today
Life
expectancy
Years of life lost
Life expectancy at the age of death
YLLs
Years of life lost to present deaths (3/3)
• 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
YLL =  N x (LE-Age)
Age groups
YLLs
Years of life lost to future deaths (1/3)
• 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
YLLs
Years of life lost to future deaths (2/3)
Year 0
Year 1
Year 2
Year 3
Life
expectancy
Death from other causes
Years of life lost
Death from other causes
Years of life lost
Death from other causes
Years of life lost
YLLs
Years of life lost to future deaths (3/3)
• 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:
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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
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
Life
expectancy
4 years lived 100%
4 years lived 75%
2 years lived 100%, 2 years lived 50%
3 years lived 100%, 1 year lived 0%
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
Life
expectancy
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 lost
DALYs
Total: 3 DALY lost
Calculating DALYs
• Calculate years of life lost through death
• Calculate years of life lived with disability
• Add the two
Year 0
Year 1
Year 2
Year 3
1 year lived 100%, 2 years lived 50%, 1 year lost: 2 DALYs lost 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
One year, 30% disability weight
One year, 50% disability weight
Life
expectancy
One year, 70% disability weight
Years of life lost
DALYs
Total: 2.5 DALYs lost
Calculating DALYs lost in the future
• 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
DALYs
Calculating DALYs lost in the future
Year 0
Year 1
Year 2
Year 3
Life
expectancy
Death from other causes
Years of life lost
Death from other causes
Years of life lost
Death from other causes
Years of life lost
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
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Allow comparisons
between diseases
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Take age at death
into account
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Take disability into
account
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