Disability- Adjusted Life Years

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Abdur Razzaque Sarker,
MHE (Health Economics) , MSS (Economics)
Health Economic and Financing Research Group
ICDDR,B
Email: razzaque.sarker@gmail.com

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 Concept of DALYs
Background
 Difference between QALYs and DALYs
 Health gaps
 Components of DALYs
 Principles of DALYs
 Practice
 Estimated DALYs in Worldwide
 Estimated DALYs in Bangladesh

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 The global burden of disease study report was released in 1996
The disability-adjusted life year (DALY) is a measure of overall diseases
burden expressed as the number of years lost due to ill-health, disability or
early death.
 The burden of disease methodology was developed by researchers at
Harvard University and the World Health Organization for the World Bank to
quantify the amount of premature mortality and disability present
in any given population.
 The summary measure that the methodology derived was the Disability
Adjusted Life Year or DALY
Background

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Differences between QALYs and DALYs
DALY and QALY are both used as measuring health
QALYs DALYs
QALY= Quality Adjusted Life Years DALY= Disability Adjusted Life Years
QALY measures the quality of life in
health gain
DALY measures health loss in the
quality of life
QALY is expressed in either 1 or 0,
Where
1 = Perfect Health
0= Death
DALY is expressed in either 0 or 1,
Where
O = Perfect Health
1= Death

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Disability Adjusted Life Years (DALYs)
 DALY is the only quantitative indicator of burden of diseases that
reflects the total year lost (pre-mature mortality) or some degree
of disability (morbidity) during a period of time.
 Premature mortality is measured as years of life lost (YLLs)
 Morbidity is measured in terms of years lived with a disability
(YLDs)
Thus , DALY = YLL + YLD
 One DALY represents one lost year of healthy life

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For example, the DALYs for road traffic accidents
in any given area is defined as the total YLLs in
fatal road accidents added to the total years of
life lived with disabilities by survivors of
accidents in the same year.

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Health Gaps: Social value choices
1. How long should people in good health expect to live?
2. Is a year of healthy life gained now worth to society than a
year of healthy life gained sometime in the future?
3. How should we compare years of life lost through death
with years lived with poor health or disability of various
levels of severity?
4. Are lost years of healthy life valued more at some ages
than others?
5. Are all people equal?

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Health Gaps: Social value choices
1. How long people in good health expect to live?
Standard life expectancy
2. Value of future years of Discounting
3. Compare years lost due to death with years lived in poor
health Disability weight
4. Value of healthy years of life at all ages.
Age weight
5. Same values for all regions of the world
All people are equal
These are the five components of DALYs

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1. Standard life expectancy
 The DALY measures health gaps as opposed to health expectancies
 It measures the difference between a current situation and an ideal
situation where everyone lives up to the age of the standard life expectancy,
and in perfect health.
 Based on life tables, the standard life expectancy at birth is set at 80
years for men and 82.5 for woman according to Japan because of highest
life expectancy
 It is better to use country specific life expectancy from a life table

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2. Discounting
 People generally prefer a healthy year of life immediately, rather than in
the future, if given the choice
 The GBD study applied a 3% time discount rate to years of life lost in the
future
 The value of health gains today compared to the value attached to health
gains in the future
Discounting function = e – r (x-a)
Where
e = Constant equal to 2.71
r = Discount rate; 3%, 5%
x = age
a = age at onset

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3. Disability weight
 In the calculation of the DALY, a weighting system is
used to measure the severity of a disability
 The disability weight is measured on a scale of zero to
one
0 1
Full health Death

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Disability classes in GBD 1990
Disability
class
Severity
weights
Indicator conditions
1 0.00 – 0.02 Vitiligo on face, weight-for –height less than 2SDs
2 0.02 – 0.12 Watery diarrhoea, severe sore throat, sever anemia
3 0.12 – 0.24 Radius fracture in stiff cast, infertility, angina,
arthritis
4 0.24 – 0.36 Deafness, below the knee amputation
5 0.36 – 0.50 Mild mental retardation, Down syndrome, Fistula
6 0.50 – 0.70 Unipolar major depression, blindness, paraplegia
7 0.70 – 1.00 Active psychosis, dementia, severe migraine

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Disease or sequelae Mean disability weight
(untreated form)
Mean disability
weight (treated form)
AIDS 0.50 0.50
Diarrhea disease, episode 0.11 0.11
Tuberculosis 0.27 0.27
Trachoma, blindness 0.60 0.49
Unipolar major depression 0.60 0.30
Cancer 0.81 0.81
Asthma, cases 0.10 0.06
Malaria, episodes 0.20 0.20
Some Diseases Specific Weight
Source: Murray & Lopez (1996)

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4. Age weight
 There is a broad social preferences to value a year
lived by a young adult more highly than a year lived
by young child or at older ages
 Not related to productivity but social role in caring
for the young and old.
Age Weighting Function = C x e – β x
Where,
C = Constant equal to 0.1658
β = Constant equal to 0.04
e = Constant equal to 2.71
x = Age

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5. Health is simply added across individuals
 Two people each losing 10 years of disability-free life are
treated as the same loss as one person losing 20 years
One could also weight duration non-linearly, so as to give
priority to fewer people suffering for long intervals over more
people suffering for shorter intervals
 It is lead -all people are equal

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Principle of DALYs
The DALY combines in one measure the time
lived with disability and the time lost due to
premature mortality:
DALY = YLL + YLD
Where,
YLL= Years of Life Lost
YLD= Years Lived with Disability

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Calculating YLL and YLD
 The basic formula for calculating the YLL
YLL = N x L
where:
N = number of deaths.
L = standard life expectancy at age of death (in
years)
 The basic formula for calculating the YLD
YLD = I x DW x L
where:
I = number of incident cases.
DW = disability weight.
L = average duration of disability (years)

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Applying with a 3% discount rate
 YLL becomes:
Where, N = number of deaths.
L = standard life expectancy at age of death (years)
r = discount rate
YLL = N * (1 – e-0.03L)
0.03
 YLD becomes:
Where, I = number of incident cases
DW= disability weight
L = duration of disability (years)
r = discount rate
YLD = I * DW * (1 – e-0.03L)
0.03

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Applying with discounting and age-weight
 The initially simple formulas for YLL and YLD
become more complicated
YLL becomes:
YLL= N KCe(ra) / (b+r)2 [e-(b+ r) (L+a) [-(b+r) (L+a)-1] - e-(b+ r)a [–(b+r)a-1]]

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YLD = I * DW * K Ce(ra) / (b+r)2 [e-(b+r)(L+a) [-(b+ r)(L+a)-1] - e-(b+ r)a [–(b+r)a-1]]
Where,
I = number of incident cases
L = duration of disability
D = disability weight
x = age
a = age at onset/ age of death
r = discount rate
C = constant
β = constant
K = constant
YLD with discounting and age weight

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Practice
Q. A woman become sick with a bipolar depression at age 35,
that she has it for 10 years and dies as a result.
How many DALYs(0.03,1,o.o4) have been lost as a result
Suppose, life expectancy is 79.13 years.

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Case 1: DAL Ys Lost Due to Immediate Death
The horizon for this case is
(10 years disable) (34.73 Years lost)
35 YLD 45 (YLL) 79.13
In this case,
L = Years lived with disability = 10
D = disability weight =0.6
a = age at onset/ age of death= 35
r = discount rate = 0.03
C = constant =0.1658
β = constant = 0.04
K = constant = 1
e = constant = 2.72

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YLD = I * DW * K Ce(ra) / (b+r)2 [e-(b+r)(L+a) [-(b+ r)(L+a)-1] - e-(b+ r)a [–(b+r)a-1]]
Case 1 : To calculate YLD
=0.6{1*0.1658*2.72(0.03*35)/(0.03+0.04)2[2.72-
(0.03+0.04)(10+35)[-(0.03+0.04)(10+35)-1]-2.72-(0.03+0.04)35[-
(0.03+0.04)35-1]]
=6.95
The number of DALY loss due to disability is 6.95

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Case 2 : To calculate YLL
It requires two steps
1. The years of life lost from age 45 onwards is
calculated
2. This value is converted to the expectation of life lost
from age 35 so all DALYs can be added up from age
35 onwards using common metric.

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The first steps requires a straightforward substitution of
values into general YLL equation using the following values
In this case,
L = Life expectancy at age 45 = 34.73
a = age at onset/ age of death= 45
r = discount rate = 0.03
C = constant =0.1658
β = constant = 0.04
K = constant = 1
e = constant = 2.72
YLL= N K Ce(ra) / (b+r)2 [e-(b+ r) (L+a) [-(b+r) (L+a)-1] - e-(b+ r)a [–(b+r)a-1]]

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YLLs [0.0,1,0.04] =
1*0.1658*2.72(0.03*35)/(0.03+0.04)2[2.72-(0.03+0.04)(34.73+45)
[-(0.03+0.04)(34.7+45)-1]-2.72-(0.03+0.04)45[-(0.03+0.04)45-1]]
=19.97
 The total DALYs lost from the age of 45 onwards, due to lost
years of life is therefore 19.97

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Second Step
The second step is to convert DALYs at age 45 (19.97) to age
at onset , 35 years with following formula
DALYs at age 35 = DALYs at age (45)× e-rs
where s is the number of years to be discounted, that is (45-35)
= 10
DALYs at age 35 = 19.97× 2.71-(0.03×10)
= 14.80
Hence, Total DALYs Lost = YLD + YLL
= 6.95+14.80
= 21.75

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Calculating DALY: Sensitivity Analysis
 Discounting at 0%, 3%, 5%
 Age weighting – yes / no
 DWs – Dutch weight, GBD weights

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We can Calculate DALYs using MS Excel

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If we consider 60-69 ages the estimated DALYs will be
DALYs = YLL (Male + Female) + YLD (Male + Female)
=(174919+196437)+(77755+116632)
=(371365+194387)
=565743

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 Using DISMOD software we can estimate the burden of
diseases.
It is a free software available in www.who website
 The original version of DISMOD is available from the
Burden of Disease Unit, Harvard University. For
downloading the software visit,
www.hsph.harvard.edu/organizations/bdu/dismod
DISMOD

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Murray CJ, Lopez AD. N Engl J Med 2013;369:448-457.

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Findings: Global Burden of Disease in DALY, 1999 (000s)

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Findings: DALY (000s), Bangladesh, 1990
Disease Category Male Female
DALY % DALY %
Respiratory 3766.4 17.2 4100.1 16.6
Perinatal causes 3240.4 14.8 3354.0 13.6
Injury 3066.0 14.0 2387.5 9.7
Diarrhoeal 2741.7 12.5 3006.7 12.2
Maternal - 0.0 2889.9 11.7
Nutrition 2156.7 9.9 2595.5 10.5
Cardiovascular 1592.5 7.3 1455.6 5.9
Neuro-Psych & Sense 1407.5 6.4 1605.5 6.5
Childhood INF 1189.5 5.4 1193.1 4.8
Congenital 541.2 2.5 550.9 2.2
Tuberculosis 796.0 3.6 456.5 1.8
Digestive 516.6 2.4 306.9 1.2
Malignant Neoplasm 433.0 2.0 497.5 2.0
Tropical Disease 281.6 1.3 127.4 0.5
Diabetes 143.4 0.7 150.0 0.6
Total 21872.5 100.0 24677.1 100.0

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Thank You All