Calculating Disability-adjusted Life Years To Quantify Burden Of . - CBRA

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Int J Public HealthDOI 10.1007/s00038-014-0552-zHINTS AND KINKSCalculating disability-adjusted life years to quantify burdenof diseaseBrecht Devleesschauwer Arie H. Havelaar Charline Maertens de NoordhoutJuanita A. Haagsma Nicolas Praet Pierre Dorny Luc Duchateau Paul R. Torgerson Herman Van Oyen Niko Speybroeck Received: 23 February 2013 / Revised: 12 February 2014 / Accepted: 3 April 2014Ó Swiss School of Public Health 2014IntroductionThe 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. 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 both.On a population level, diseases with a higher public healthimpact will thus account for more DALYs than those witha lesser impact.B. Devleesschauwer (&) C. Maertens de Noordhout N. SpeybroeckFaculty of Public Health, Institute of Health and Society (IRSS),Université catholique de Louvain, Brussels, Belgiume-mail: Brecht.Devleesschauwer@UGent.beB. Devleesschauwer P. 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 NetherlandsDALYs 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 Lopez 1996;Lopez et al. 2006; World Health Organization 2008;Murray et al. 2013a; Lopez 2013). Table 1 shows the mostimportant diseases according to the different GBD studies.Furthermore, various national and regional DALY calculations have been performed to assess and monitor localhealth and to set priorities within the local health sector(e.g., Melse et al. 2000; Mathers et al. 2001; Devleesschauwer et al. 2013; Shield et al. 2013).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. TheN. 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. R. TorgersonSection of Epidemiology, Vetsuisse Faculty,University of Zürich, Zurich, SwitzerlandH. Van OyenUnit of Public Health and Surveillance, Scientific Institute ofPublic Health, Brussels, BelgiumA. H. HavelaarInstitute for Risk Assessment Sciences, University Utrecht,Utrecht, The NetherlandsJ. A. HaagsmaDepartment of Public Health, Erasmus MC, University MedicalCenter Rotterdam, Rotterdam, The Netherlands123

B. Devleesschauwer et al.Table 1 Top three diseases according to the different Global Burden of Disease studiesReference yearMurray and Lopez (1996)Lopez et al. (2006)World Health Organization (2008) Murray et al. (2013a)199020012004Top three diseases Lower respiratory infections Perinatal conditionsLower respiratory infectionsIschaemic heart diseasePerinatal disordersLower respiratory infections Unipolar depressive disordersBasic formulaeDALYs are composed of years lived with disability(YLDs) and years of life lost due to premature mortality(YLLs). YLDs, the morbidity component of the DALYs,are calculated as followsYLD ¼ Number of cases duration till remission or death disability weightð1ÞYLD ¼ 1 ð60 40Þ 0:55 ¼ 11YLLs, the mortality component of the DALYs, arecalculated as follows:YLL ¼ Number of deaths life expectancy at the age of deathStrokeWest. This table has a life expectancy at birth of 80 for malesand 82.5 for females (Murray 1994). 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 al. 2013b). Alternatively, local lifetables may be used instead of standard life tables.Continuing 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 ¼ 1 25 ¼ 25DALYs are simply the sum of the YLDs and YLLs: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. Living 10 years witha DW of 0.10, or 5 years with a DW of 0.20, 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 0.55 (Salomon et al. 2013), 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:ð2ÞThe first three GBD studies use the life expectancy at theage of death from the Coale–Demeny Model Life Table123Ischaemic heart diseaseDiarrhoeal diseasesDiarrhoeal diseasescompanion paper (Devleesschauwer et al. 2014) 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 al. 2013).2010Lower respiratory infectionsDALY YLD YLLð3ÞFor our patient, this would mean 11 YLDs ? 25YLLs 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 DALYs.Social weightingThe basic formulae for YLDs and YLLs may be extendedby applying so-called social weighting functions. Thisimplies that not all life years lost will be valued equally,and social weighting is therefore not universally accepted(Barendregt et al. 1996; Arnesen and Kapiriri 2004.Age 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 (Murray 1994).The standard age weighting formula is:Cxe bxð4Þ

Calculating disability-adjusted life yearsTable 2 Years lived with disability (YLDs), years of life lost (YLLs)and disability-adjusted life years (DALYs) for the alcohol use disorder example under different social value choicesScenario [K; r]AgeweightingDiscountrate (%)YLDYLLDALYDALY [0; 0]No011.025.036.0DALY [1; 0]Yes012.316.729.1DALY [0; 0.03]No38.39.717.9DALY [1; 0.03]Yes39.56.716.2with x the concerned age, and C and b constants commonlyset to 0.1658 and 0.04.Time discounting discounts years of healthy life livedin the future at a rate of (usually) 3 %. This reflectssimilar practices in economic assessments, and wouldprevent policy makers from saving resources for a futureeradication program, instead of investing in currentlyavailable, but less effective, intervention measures (Murray 1994).The standard time discounting formula is as follows:e rðx aÞð5Þwith r the discount rate, x the concerned age, and a the ageto which the burden will be assigned.Combining both social weighting functions gives thefollowing extended YLD and YLL formulae:YLD ¼ N DW ZAþLnoKCxe bx e rðx aÞ þ ð1 KÞe rðx aÞ dxAð6ÞZAþLnoKCxe bx e rðx aÞ þ ð1 KÞe rðx aÞ dxYLL ¼ M Að7Þwhere N equals the number of cases, M the number ofdeaths, and DW the disability weight. K is a modulatingfactor equalling one if age weighting is applied and zerootherwise; A and L represent, respectively, the age at onsetand the duration (Eq. 6), or the age at death and the lifeexpectancy at the age of death (Eq. 7).The subjective choices of whether or not to apply ageweighting and time discounting can have important effects onthe DALY estimates. Only DALYs based on the same socialvalue choices should therefore be compared. For illustration,we calculated DALYs for our example under different scenarios (Table 2; ‘‘Appendix’’). Figure 1 visualizes theseresults (note that the integrals of Eqs. (6) and (7) translate toareas under the curve). Table 3 presents the social valuechoices applied by the four different GBD studies.Given the lack of consensus on social weighting, werecommend to calculate DALYs under different scenarios.By calculating at least the DALY [0; 0], DALY [0; 0.03],and DALY [1; 0.03] scenario, comparability with mostother studies is likely.Presenting DALYsDALYs can be expressed as an absolute number, therebygiving an idea of the total population burden. They can also beexpressed relative to the population, e.g., as the number ofDALYs per 1,000 population. This enables a directFig. 1 Years lived withdisability (YLDs) and years oflife lost (YLLs) for the alcoholuse disorder example underdifferent social weightingscenarios. The top left plot is thebasic disability-adjusted lifeyear (DALY) calculation. Thebottom left plot includes ageweighting; the curved black lineis the age-dependent zerodisability level, while thestraight grey line compares thesituation without age weighting.The top right plot includes a3 % time discount rate; theburden is assigned to the year ofdisease onset (i.e., the age of40). The bottom right plot,finally, combines age weightingand a 3 % time discount rate123

B. Devleesschauwer et al.Table 3 Social value choices applied by the different Global Burden of Disease (GBD) studiesStudyAge weightingDiscount rate (%)Scenario [K; r]GBD 1990: Murray and Lopez (1996)Yes3DALY [1; 0.03]GBD 2001: Lopez et al. (2006)No3DALY [0; 0.03]GBD 2004: World Health Organization (2008)Yes3DALY [1; 0.03]GBD 2010: Murray et al. (2013a)No0DALY [0; 0]comparison of the burden suffered by different populations.Finally, DALYs may also be expressed relative to the numberof cases. This allows comparisons of the impact of diseases atthe patient-level, instead of at the population level.All assumptions used in the DALY calculation shouldbe explicitly mentioned, including DWs, life expectancytable and social weighting functions. This ensures correct123interpretation of DALY estimates and reduces ‘‘error’’variation between burden studies (Polinder et al. 2012).Appendix: calculating DALYs in RThe following R code can be used to calculate DALYs:

Calculating disability-adjusted life yearsReferencesArnesen T, Kapiriri L (2004) Can the value choices in DALYsinfluence global priority-setting? Health Policy 70:137–149Barendregt JJ, Bonneux L, Van der Maas PJ (1996) DALYs: the ageweights on balance. Bull World Health Organ 74:439–443Bjegovic-Mikanovic V, Vukovic D, Otok R, Czabanowska K, LaaserU (2013) Education and training of public health professionals inthe European Region: variation and convergence. Int J PublicHealth 58:801–810Devleesschauwer B, Ale A, Torgerson P, Praet N, Maertens deNoordhout C, Pandey BD, Pun SB, Lake R, Vercruysse J, JoshiDD, Havelaar AH, Duchateau L, Dorny P, Speybroeck N (2013)The burden of parasitic zoonoses in Nepal: a systematic review.PLoS Negl Trop Dis 8:e2634Devleesschauwer B, Havelaar AH, Maertens de Noordhout C,Haagsma JA, Praet N, Dorny P, Duchateau L, Torgerson PR,Van Oyen H, Speybroeck N (2014) DALY calculation inpractice: a stepwise approach. Int J Public Health. doi:10.1007/s00038-014-0553-yLopez AD (2013) Reducing risks to health: what can we learn fromthe Global Burden of Disease 2010 Study? Int J Public Health58:645–646Lopez AD, Mathers CD, Ezzati M, Jamison DT, Murray CJL (2006)Global burden of disease and risk factors. Oxford UniversityPress, New YorkMathers CD, Vos ET, Stevenson CE, Begg SJ (2001) The burden ofdisease and injury in Australia. Bull World Health Organ79:1076–1084Melse JM, Essink-Bot ML, Kramers PG, Hoeymans N (2000) Anational burden of disease calculation: Dutch disability-adjustedlife-years. Dutch Burden of Disease Group. Am J Public Health90:1241–1247Murray CJL (1994) Quantifying the burden of disease: the technicalbasis for disability-adjusted life years. Bull World Health Organ72:429–445Murray CJL, Lopez AD (1996) The Global Burden of Disease: acomprehensive assessment of mortality and disability fromdiseases, injuries and risk factors in 1990 and projected to 2020.Harvard University Press, CambridgeMurray CJ, Vos T, Lozano R et al (2013a) Disability-adjusted lifeyears (DALYs) for 291 diseases and injuries in 21 regions,1990–2010: a systematic analysis for the Global Burden ofDisease Study 2010. Lancet 380:2197–2223Murray CJ, Ezzati M, Flaxman AD, Lim S, Lozano R, Michaud C,Naghavi M, Salomon JA, Shibuya K, Vos T, Wikler D, LopezAD (2013b) GBD 2010: design, definitions, and metrics. Lancet380:2063–2066Polinder S, Haagsma JA, Stein C, Havelaar AH (2012) Systematicreview of general burden of disease studies using disabilityadjusted life years. Popul Health Metr. 10:21Salomon JA, Vos T, Hogan DR et al (2013) Common values inassessing health outcomes from disease and injury: disabilityweights measurement study for the Global Burden of DiseaseStudy 2010. Lancet 380:2129–2143Shield KD, Kehoe T, Taylor B, Patra J, Rehm J (2013) Alcoholattributable burden of disease and injury in Canada, 2004. Int JPublic Health 57:391–401World Health Organization (2008) The global burden of disease: 2004update. http://www.who.int/healthinfo/global burden disease/2004 report update/en/index.html. Accessed 23 Feb 2013123

deaths, and DW the disability weight. K is a modulating factor equalling one if age weighting is applied and zero otherwise; A and L represent, respectively, the age at onset and the duration (Eq. 6), or the age at death and the life expectancy at the age of death (Eq. 7). The subjective choices of whether or not to apply age

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