Calculating disability-adjusted life years to quantify ...

1 HINTS AND KINKSC alculating disability - adjusted life years to quantify burdenof diseaseBrecht Devleesschauwer Arie H. Havelaar Charline Maertens de Noordhout Juanita A. Haagsma Nicolas Praet Pierre Dorny Luc Duchateau Paul R. Torgerson Herman Van Oyen Niko SpeybroeckReceived: 23 February 2013 / Revised: 12 February 2014 / Accepted: 3 April 2014 Swiss School of Public Health 2014 IntroductionThe disability - adjusted life year or DALY is a summarymeasure of public health widely used to quantify burden ofdisease. In the DALY philosophy, every person is bornwith a certain number of life years potentially lived inoptimal health.

2 People may lose these healthy life yearsthrough living with illness and/or through dying before areference life expectancy . These losses in healthy life yearsare exactly what is measured by the DALY metric. TenDALYs, for instance, correspond to ten lost years ofhealthy life , attributable to morbidity, mortality, or a population level, diseases with a higher public healthimpact will thus account for more DALYs than those witha lesser have been the key measure in the four GlobalBurden of Disease (GBD) studies, each presenting acomprehensive assessment of the worldwide health impactof disease, injury and risk factors (Murray and Lopez1996;Lopez et ; World Health Organization2008;Murray et ; Lopez2013).

3 Table1shows the mostimportant diseases according to the different GBD , various national and regional DALY calcu-lations have been performed to assess and monitor localhealth and to set priorities within the local health sector( , Melse et ; Mathers et ; Devleess-chauwer et ; Shield et ).In this Hints and Kinks paper, we summarize the DALY sbasic features and present a description of its calculation. An Appendix includes R code to calculate DALYs. TheB. Devleesschauwer (&) C. Maertens de Noordhout N. SpeybroeckFaculty of Public Health, Institute of Health and Society (IRSS),Universite catholique de Louvain, Brussels, Belgiume-mail: Devleesschauwer P.

4 DornyDepartment of Virology, Parasitology and Immunology,Faculty of Veterinary Medicine, Ghent University,Merelbeke, BelgiumA. H. HavelaarCentre for Infectious Disease Control, National Institute forPublic Health and the Environment (RIVM), Bilthoven,The NetherlandsA. H. HavelaarInstitute for Risk Assessment Sciences, University Utrecht,Utrecht, The NetherlandsJ. A. HaagsmaDepartment of Public Health, Erasmus MC, University MedicalCenter Rotterdam, Rotterdam, The NetherlandsN. Praet P. DornyDepartment of Biomedical Sciences, Institute of TropicalMedicine, Antwerp, BelgiumL. DuchateauDepartment of Comparative Physiology and Biometrics,Faculty of Veterinary Medicine, Ghent University,Merelbeke, BelgiumP.

5 R. TorgersonSection of Epidemiology, Vetsuisse Faculty,University of Zu rich, Zurich, SwitzerlandH. Van OyenUnit of Public Health and Surveillance, Scientific Institute ofPublic Health, Brussels, BelgiumInt J Public HealthDOI paper (Devleesschauwer et ) presents astepwise approach towards conducting a DALY assessmentstudy. These papers may serve as a guide and reference forpublic health professionals interested in quantifying burdenof disease (Bjegovic-Mikanovic et ).Basic formulaeDALYs are composed of years lived with disability (YLDs) and years of life lost due to premature mortality(YLLs).

6 YLDs, the morbidity component of the DALYs,are calculated as followsYLD Number of cases duration till remission or death disability weight 1 The disability weights (DWs) are a crucial component ofthe DALY calculation, as they translate morbidity intohealthy life years lost, thus enabling comparison ofmorbidity and mortality. A DW, scaled from zero(perfect health) to one (worst possible health state), canbe interpreted as the proportional reduction in good healthdue to an adverse health state. In the DALY mindset, this isset equivalent to losing the same proportion of healthy lifeyears over the course of the disability .

7 Living 10 years witha DW of , or 5 years with a DW of , thus bothcorrespond to losing one full healthy life year . Forexample, a female patient develops severe alcohol usedisorder at age 40 and consequently dies at age 60. Thiscondition has a DW of (Salomon et ), and isthus assumed to cause a 55 % reduction of good health, or,equivalently, a loss of 55 % of the potential healthy lifeyears lived during the 20 years of suffering from thiscondition. The number of YLDs for this patient is thereforecalculated as:YLD 1 60 40 0:55 11 YLLs, the mortality component of the DALYs, arecalculated as follows:YLL Number of deaths life expectancy at the age of death 2 The first three GBD studies use the life expectancy at theage of death from the Coale Demeny Model life TableWest.

8 This table has a life expectancy at birth of 80 for malesand for females (Murray1994). In the latest GBD study,a new model life table was developed, with a life expectancyat birth of 86 for both males and females (available as asupplement in Murray et ). Alternatively, local lifetables may be used instead of standard life the aforementioned example, the lifeexpectancy of 60- year -old females are 25 years in theCoale Demeny Model life table West. Dying at the ageof 60 will thus cause a loss of 25 full life years potentiallylived in optimal health:YLL 1 25 25 DALYs are simply the sum of the YLDs and YLLs:DALY = YLD + YLL 3 For our patient, this would mean 11 YLDs?

9 25 YLLs=36 DALYs, which can be interpreted as a loss of36 healthy life years . These 36 years (DALYs) are composedof the equivalent of 11 full healthy life years lost due to livingin a less-than-optimal health state (YLDs), and the actual 25healthy life years lost due to premature death (YLLs).In population-based burden studies, average DALYs arecalculated for specific age and sex strata, based on the totalnumber of cases and deaths in each stratum, and theaverage duration, age at onset and age at death in eachstratum. Population totals are then obtained by summingthese stratum-specific weightingThe basic formulae for YLDs and YLLs may be extendedby applying so-called social weighting functions.

10 Thisimplies that not all life years lost will be valued equally,and social weighting is therefore not universally accepted(Barendregt et ; Arnesen and weighting, as used in the initial GBD study andmany ensuing studies, implies that the value of life dependson age. A higher weight is given to the healthy life yearslived in the (assumed) socially more important life spanbetween 9 and 56 (Murray1994).The standard age weighting formula is:Cxe bx 4 table 1 Top three diseases according to the different Global Burden of Disease studiesMurray and Lopez(1996)Lopez et al.)