Estimating disability-adjusted life years

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Disability-adjusted life years (DALYs) are a method for combining different health impacts such as mortality and morbidity into a single common metric. The DALY is one of the most commonly used integrated health measures and was first introduced by Murray and Lopez (1996) in collaboration with World Health Organization and the Worldbank in an attempt to introduce morbidity in mortality-based health discussions. In effect, the DALY integrates many dimensions of public health impact e.g. the number of persons affected by a particular agent or event, the severity and duration of any health effects.

This document describes briefly how Disability Adjusted Life Years (DALYs) can be used in a Health Impact Assessment (HIA). After a short introduction about integrated health measures, the calculation steps are described. Together with Excel sheets made for this purpose (“DALY worksheets Intarese”), this information can be used as a basis for the calculation of environmental DALYs.

The proponents of DALY's use the metric for atleast two separate exercise

1. Positive exercise of measuring the burden of disease.

2. The normative exercise for resource allocation.

The burden of disease is simply measured as the sum of DALYs attributable to premature mortality and morbidity. for resource allocation, Murrey suggested that DALYs be used "in conjunction with the literature on cost effectiveness of health intervention" so as "using estimates of the burden of disease in determining health resource allocations". In using DALY for this purpose, the object is to minimize and aggregate DALYs subject to a given budget.

Murrey states "{T}he he intended use of an indicator of the burden of disease is critical to its design. at least 4 objectives are very important

• to aid in sitting health service (both curative and preventive) priorities.

• to aid in sitting health research priorities.

• to aid in identifying disadvantaged group and targeting of health intervention

• to to provide a comparable measure of output for intervention, program and sector evaluation and planning.

Not everyone appreciate the ethical dimension of heath status indicator. Nevertheless, the first two objectives listed for measuring burden of disease could influence the allocation of resource among individuals, clearly establishing an ethical dimension to the construction of an indicator of the burden of disease.

Integrated health measures

Health effects of environmental factors can vary considerably with regard to their severity, duration and magnitude. This makes it difficult to compare different (environmental) health effects. An integrated health measure, using the same denominator for all health effects, can help with interpretation and comparison of health problems and policies. They quantify and summarize (environment-related) health effects and can be used for:

• Comparative evaluation of environmental health impacts (“how bad is it?”)
• Evaluation of the effectiveness of environmental policies (largest reduction of disease burden)
• Estimation of the accumulation of exposures to environmental factors (for example in urban areas)
• Communication of health risks

An example of an integrated health measure is the DALY (Disability Adjusted Life Years). DALYs combine information on quality and quantity of life. They give an indication of the (potential) number of healthy life years lost due to premature mortality or morbidity. In these calculations, morbidity is weighted for the severity of the disorder.

Calculation

DALY in case of recovery:

Peffect  * (Prec* YLDrec * Sev)

DALY in case of death

Peffect * [Pdie *(YLDdie * Sev) + L.E. - C.A. - YLDdie]

DALY if 'diseased' to natural life expectancy (chronic disease)

Peffect * (1 - Prec - Pdie) * (L.E. - C.A.) * Sev

Where:

• Peffect = Probability of the considered effect (dose-response relationship)
• Prec = Probability to recover from a disease
• Pdie = Probability to die from a disease
• YLDrec = Years Lived with Disability in case of full recovery
• YLDdie = Years Lived with Disability in case of death
• Sev = Severity of the effect (0-1)
• L.E. = Life expectancy
• C.A. = Current Age

Calculating DALYs

DALYs can be calculated using the Excel sheets provided for the Intarese project (pilot version available only).

The formula of a DALY is as follows:

• Number of people with environment-related morbidity or mortality x
• Severity factor (0 = healthy, 1 = death) x
• Duration (YLL for mortality)

A more detailed formula is given below :

DALY = AB * D * S

AB = AR * P * F

AR = (RR’-1)/RR’

RR’ = ((RR-1) * C) + 1

where:

• AB: Attributable Burden; the number of people in a certain health state as a result of exposure to the (environmental) factor that is being analyzed, not corrected for comorbidity.
• D: Duration of the health state; for morbidity, prevalence numbers have been used and therefore duration is one year (except for hospital visits, for which the mean duration of the specific hospital visit has been used). For mortality, the duration of time lost due to premature mortality is calculated using standard expected years of life lost with model life tables.
• S: Severity; the reduction in capacity due to morbidity is measured using severity weights. A weight factor, varying from 0 (healthy) to 1 (death), is determined by experts (clinicians, researchers, etc).
• AR: Attributive Risk; risk of getting a specific disease as a result of exposure to a certain (environmental) factor.
• P: Base prevalence for morbidity; number of deaths for mortality
• F: Fraction of the population exposed to the (environmental) factor under investigation (for air pollution, this fraction is set to 1, meaning that everybody is exposed to a certain degree)
• RR’: Adjusted Relative Risk
• RR: Relative Risk
• C: Concentration of the environmental factor, expressed in the unit of the Relative Risk

Discounting

DALY calculations can also include discounting factors. In discounting, future years of healthy life lived are valued less than present years (discounting normally 3%), or years lived by people in a certain age group (productive ages) are valued more than years lived by the very old and young. Ethical questions can be raised with regard to the use of these factors, and they are currently not included in the calculation sheets. They might become optional in newer versions of DALY calculation tools. More information and templates can be found at WHO health info.