DALY combines mortality and morbidity 

The burden of disease (The burden of disease is expressed in Disability Adjusted Life Years (DALY) and is made up of the number of years of life lost (due to premature death), and the number of (healthy) years due to health problems (e.g. an illness), weighted for the severity of this(disability weights). 'Burden of Disease' quantifies the health loss in a population that is caused by diseases. The burden of disease is expressed in DALY Disability Adjusted Life Years. Maat voor ziektelast ('burden of disease') in een populatie; opgebouwd uit het aantal verloren levensjaren (door vroegtijdige sterfte), en het aantal jaren geleefd met gezondheidsproblemen (bijvoorbeeld een ziekte), gewogen voor de ernst hiervan (ziektejaarequivalenten). (Disability Adjusted Life Years. Maat voor ziektelast ('burden of disease') in een populatie; opgebouwd uit het aantal verloren levensjaren (door vroegtijdige sterfte), en het aantal jaren geleefd met gezondheidsproblemen (bijvoorbeeld een ziekte), gewogen voor de ernst hiervan (ziektejaarequivalenten).) (Disability Adjusted Life Years). The DALYs is constructed out of the number of years of life lost (due to premature death, YLL Year(s) of life lost (Year(s) of life lost)), and the number of years lost due to disability (YLD). The concept originates from the World Bank and WHO Global Burden of Disease (GBD) study (Murray & Lopez, 1996 Murray, C. J. L., Lopez, A. D., The global burden of disease: a comparative assessment of mortality and disability from disease, injuries, and risk factors in 1990 and projected to 2020, Massachusetts (1996) (Murray, C. J. L., Lopez, A. D., The global burden of disease: a comparative assessment of mortality and disability from disease, injuries, and risk factors in 1990 and projected to 2020, Massachusetts (1996))). 

RIVM Rijksinstituut voor Volksgezondheid en Milieu (Rijksinstituut voor Volksgezondheid en Milieu) has been making estimates of the burden of disease in the Netherlands since 1997. Since then, the method for doing these calculations has been adapted and improved several times. For example, the number of diseases has continued to expand, a method for correction for multimorbidity has been developed, and the disability weights have recently been updated. As a result of these adjustments, the results published over different years cannot be properly compared. 

Years lost due to disability (YLD) 

The years lost due to disability (YLD) are calculated by combining the point prevalence of the disease (e.g. the number of cases or individuals with a particular disease at a given time) (number of people who have the disease at a given time) by a disability weight for the severity of the disease. The YLD Year(s) lived with disability (Year(s) lived with disability) can then be combined with the years of life lost due to mortality. For example, if a disease has a disability weight of 0.5, this means that a year of living with this disease is considered equivalent to half a year lost due to premature death.

Premature mortality expressed in years of life lost (YLL)

The years lost due to premature death are expressed in years of life lost ('Years of Life Lost', YLL). Years of life lost are calculated by multiplying the number of cause-specific deaths per year by the remaining life expectancy at the age of death.

DALYs: sum of year-of-disease equivalents and years of life lost

Burden of disease is expressed in DALYs, the sum of year-to-disease equivalents and years of life lost.

Burden of disease calculation in formulas

  1. YLD = P x DW (P = point prevalence; DW = disability weight)
  2. YLL = N x L (N = number of deaths; L = life expectancy at the time of death)
  3. DALY = YLD + YLL

Calculation of burden of disease usually based on point prevalence

For most diseases, the point prevalence has been used to calculate the disease year equivalents. For different types of cancer, the 10-year prevalence has been used. This means the number of people who developed cancer in the course of the ten years prior to the reference date and were still alive on the reference date. For a number of diseases that are usually characterized by a relatively short duration, the prevalence (The number of cases or persons with a certain disease at a certain time (point prevalence), at some point in life (lifetime prevalence) or in a certain period, for example per year (period prevalence), is not absolute or relative.) but the number of new cases used to calculate the year-to-day equivalents.  The number of new cases is multiplied by the duration of the disease.

Point prevalence of most diseases estimated from health care registrations

For many diseases, the point prevalence is estimated on the basis of data from health care registrations, such as the Nivel Primary Care Registrations for primary care. Since the nursing home population is not part of the Nivel Primary Care Registrations for primary care, an estimates prevalence of dementia has been corrected for this in nursing homes has been included for dementia. Figures on cancer (neoplasms) and injuries come from specific registries, respectively, the Dutch Cancer Registry and the Injury Information System. The prevalence of some mental illnesses has been estimated on the basis of data from population studies, such as NEMESIS-3. 

Learn more

  1. Injury Information System
  2. NEMESIS-3(external link)
  3. Nivel Care Registration
  4. Netherlands Cancer Registry

Correction for multimorbidity

It is increasingly common for people to have more than one health condition at the same time. This is also called multimorbidity. Multimorbidity occurs more frequently in old age. For people who have more than one disease at a time, the disability weight of the combination of conditions is usually smaller than the sum of separate disability weights. Previously, multimorbidity was not corrected for when calculating the burden of disease. This led to an overestimation of the burden of disease (Hilderink et al., 2016 Hilderink, H.B.M., Plasmans, MH. D., Snijders, B.E., Boshuizen, H.C., Poos, M. J. J. C., van Gool, C.H., Accounting for multimorbidity can affect the estimation of the Burden of Disease: a comparison of approaches (2016) (Hilderink, H.B.M., Plasmans, MH. D., Snijders, B.E., Boshuizen, H.C., Poos, M. J. J. C., van Gool, C.H., Accounting for multimorbidity can affect the estimation of the Burden of Disease: a comparison of approaches (2016))). Since 2018, The Dutch burden of disease calculations include a correction for multimorbidity.

No weighting for age

GBD Global burden of disease studie (Global burden of disease studie) studies originally also opted for age weighting. This is based on the principle that a year of life of a young adult outweighs a year of life of a child or an older adult (Murray & Lopez, 1996 Murray, C. J. L., Lopez, A. D., The global burden of disease: a comparative assessment of mortality and disability from disease, injuries, and risk factors in 1990 and projected to 2020, Massachusetts (1996) (Murray, C. J. L., Lopez, A. D., The global burden of disease: a comparative assessment of mortality and disability from disease, injuries, and risk factors in 1990 and projected to 2020, Massachusetts (1996))). When calculating the burden of disease, a relatively larger weight is assigned to the severity of a disease in young adulthood. In the Dutch calculations, such a weighting for age has not been applied.

No discounting of future health loss

The GBD studies originally applied the principle that a year of life now counts more than a year of life in the future (Murray & Lopez, 1996 Murray, C. J. L., Lopez, A. D., The global burden of disease: a comparative assessment of mortality and disability from disease, injuries, and risk factors in 1990 and projected to 2020, Massachusetts (1996) (Murray, C. J. L., Lopez, A. D., The global burden of disease: a comparative assessment of mortality and disability from disease, injuries, and risk factors in 1990 and projected to 2020, Massachusetts (1996))). This is a standard starting point in economic calculations and is called discounting. The Dutch calculations do not include discounting. For some purposes (e.g. cost-effectiveness analyses) it can be useful to use a discount. This may then be added afterwards.

Selection of diseases

The calculations of the burden of disease are based on the diseases selected for the Public Health Foresight Study (PHFS) (In the Public Health Future Foresight Study (PHFS), RIVM reports every four years on the development of public health in the Netherlands.)). These diseases have been selected on the basis of their contribution to mortality, prevalence, incidence and health care expenditures. In addition, diseases that are avoidable through primary or secondary prevention have also been selected. This results in a selection of over 100 (groups of) diseases.

ICD-Chapters

Estimates of the burden of disease of major groups of diseases have also been made, based on ICD-10 International Classification of Diseases, tenth revision (International Classification of Diseases, tenth revision) chapters. In addition to the burden of disease of the selected diseases, an estimate was also made of the burden of disease of the other diseases within the disease group for each main group. CBS Centraal Bureau voor de Statistiek (Centraal Bureau voor de Statistiek) causes of death statistics includes the total mortality for each ICD-10 chapter by age. This calculates the number of years of life lost per ICD-10 chapter. The number of year-off disease equivalents per ICD-10 chapter is not so easy to calculate. For the diseases that have not been selected, prevalence figures are usually missing and no disability weight is usually available. In order to estimate the number of disease year equivalents per ICD-10 chapter, additional estimates have been made for the non-selected diseases. The year-to-disease equivalents (YLDs) calculated for the VTV In de Volksgezondheid Toekomst Verkenning (VTV) rapporteert het RIVM elke vier jaar over de ontwikkeling van de volksgezondheid in Nederland. (In de Volksgezondheid Toekomst Verkenning (VTV) rapporteert het RIVM elke vier jaar over de ontwikkeling van de volksgezondheid in Nederland.) selection of diseases and the years of life lost (YLLs) calculated with the registered mortality from diseases that are not part of the selection were used for the estimation. The following assumptions were made:

  • There are two diseases in the VTV selection (dementia and stroke) that fall into two different ICD-10 chapters. Here the assumption has been made that the distribution of the YLDs over the two ICD-10 chapters corresponds to the distribution of the YLLs.
  • In ICD-10 chapters in which diseases from the PHFS selection are represented, the assumption has been made that the ratio of the YLDs and YLLs of these diseases is equal to the ratio of the YLDs and YLLs of the other diseases within the same ICD-10 chapter.
  • Some ICD-10 chapters do not include any PHFS condition. Therefore, the ratio of the YLDs and YLLs cannot be calculated for this. For ICD International Classification of Diseases (International Classification of Diseases) (International Classification of Diseases) chapters in which no diseases from the PHFS selection are represented, the assumption has been made that the ratio of the YLDs and YLLs of all PHFS diseases together is equal to the ratio of the YLDs and YLLs in the relevant ICD-10 chapter.

Calculation of the contribution of risk factors to disease burden

In addition to calculating the burden of disease, it is also important to examine which risk factors are responsible for this burden. To attribute the disease burden to risk factors, the population attributable fraction (PAF) is used. The PAF indicates what percentage of the disease burden can be attributed to certain risk factors such as smoking or physical inactivity.

The health loss (for example, expressed in DALYs) attributable to a risk factor (such as smoking or insufficient physical activity) is estimated by assessing what percentage of the prevalence and the mortality from a disease would decrease if everyone had the most favourable level of the risk factor, such as 'no one smokes' or 'everyone is sufficiently physically active.'

To calculate the contribution of risk factors to the disease burden, the Population Attributable Fraction (PAF) is used. For each disease, this is based on:

  • the prevalence of the risk factor in the population, for example, the percentage of smokers;
  • the increased risk of developing and/or dying from the disease due to exposure to a risk factor, the relative risk, for example, smokers have a 20 times higher risk of getting lung cancer compared to never-smokers;
  • the optimal level of the risk factor, for example, for smoking, people who have never smoked.

Learn more

l

  • H.B.M. Hilderink (RIVM)
  • M.J.J.C. Poos (RIVM)
  • M.H.D. Plasmans (RIVM)
  • P.E.D. Eysink (RIVM)
  • M. Buijs (RIVM)