QALYs and DALYs are both common outcome measures in economic evaluations of health interventions. ... increased interest in the development, calculation.
Malaysian Journal of Public Health Medicine 2011, Vol. 11(2): 27-31
MINI REVIEW MEASURES OF POPULATION HEALTH: GENERAL PERSPECTIVE ON QUALITY ADJUSTED LIFE YEARS (QALYs) AND DISABILITY ADJUSTED LIFE YEARS (DALYs) Natrah S1, Sharifa Ezat WP1 1
Department of Community Medicine, National University of Malaysia Medical Centre.
ABSTRACT Impact of health care on the population health has been measured in terms of morbidity and mortality but this measurement doesn’t distinguish between children, adults and the elderly. It does not also take into account the losses that occur because of handicap, pain, or other disability. Therefore, measures of population health which combine information on mortality and non-fatal health outcomes to represent the health of a particular population as a single number was introduced. QALYs and DALYs are both common outcome measures in economic evaluations of health interventions. QALYs is the comprehensive measure of health outcome because it can simultaneously capture gains from reduced morbidity (quality gains) and reduced mortality (quantity gains) and combine these into a single measure. DALYs is primarily a measure of disease burden where it combines losses from premature death and loss of healthy life resulting from disability. Although QALYs and DALYs are almost similar in their basic concept but there are few distinct differences which must be paid attention to in order to correctly utilize these measures. Key words: Measure of population health, Quality Adjusted Life Years (QALYs), Disability Adjusted Life Years (DALYs).
INTRODUCTION Being in a good health, as people know from their own experience, is a crucial part of well-being, but spending on health can also be justified on purely economic grounds. According to World Bank in its World Development Report 1993, improved health contributes to economic growth in four ways whereby production losses can be reduced through worker improvement of their health; it permits the use of natural resources that had been totally or nearly inaccessible because of disease; children enrollment in school will be increase and makes them better able to learn; and it frees for alternative uses resources that would otherwise have to be spent on treating illness 1. A nation with a good population health will definitely succeed in the economic and social development. Impact of health care to the population health traditionally has been measured in terms of mortality, for example deaths averted. A potential drawback of using deaths averted to measure health effects is that the method doesn’t distinguish between children, adults and the elderly2. That indicator also fails to account for the losses that occur this side of death because of handicap, pain, or other disability1. Therefore,
measures of population health which combine information on mortality and non-fatal health outcomes to represent the health of a particular population as a single number was introduced. In the past decade there has been a markedly increased interest in the development, calculation and use of these measures so called Quality adjusted life years (QALYs) and Disability adjusted life years (DALYs). QALYs and DALYs are both common outcome measures in economic evaluations of health interventions. Apart from combining mortality with morbidity in single numerical units, they are an exercise involving trade-offs between quantity for quality of health. Measures of population health Internationally, the development of measures of population health has given rise to two different movements. One (in Denmark and the European Union) aims to develop a large number of standardised indicators, while the other (the World Health Organization and the World Bank) builds on the idea that it must be possible to combine the many indicators into a single summary measure of a nation's state of health. In the summary measures, distinction is made between health expectancies and health gap measures3. According to World Health Organization (WHO) and World Bank,
Malaysian Journal of Public Health Medicine 2011, Vol. 11(2): 27-31 summary measures of population health can be divided into two groups which are health expectancies and health gaps. Health expectancies are measures which take into account some lower weights for years lived in health states worse than full health while health gaps quantifies the difference between the actual health of a population and some stated norm or goal for population health4. Summary measures of population health are useful in many ways. According to Murray4, summary measures of population health can be use for: Comparing the health status of one population with another. This kind of comparisons are an essential input into evaluations of the performance of different health systems, along with information on health inequalities, responsiveness and fairness in financing. Comparisons may allow decisionmakers to focus their attention on health systems with the worst achievement for a given level of resources. Comparative judgments’ also provide the dependent variable in analyses of the independent variables that contribute to health differences between populations. Monitoring health of a population in which health status of a given population can be monitored over time and this is essential in the evaluation of health system performance and progress towards stated goals for a given society. Identifying and quantifying overall health inequalities within populations. Providing appropriate and balanced attention to the effects of non-fatal health outcomes on overall population health, without summary measures, conditions that cause decrements in function but not mortality per se tend to be neglected in favor of conditions that primarily cause mortality. Informing debates on priorities for health service delivery and planning. A summary measure can be combined with information on the contributions of different causes of disease and injury or risk factors to the total. Such information should be a critical input to debates on the identification of a short list of national health priorities that will receive the attention of senior managers in public health agencies and of government leaders. Informing debates on priorities for research and development. The relative contributions of different diseases, injuries and risk factors to the total summary measure also represent a major input to the debate on priorities for investment in research and development
Improving curricula for professional training in public health. Analyzing the benefits of health interventions for use in cost-effectiveness analyses. The change in some summary measure of population health offers a natural unit for quantifying intervention benefits in these analyses. To be more specific, Murray4 outlined 4 aims in developing an indicator measuring burden of disease in a population using measures of population health namely Disability Adjusted Life Years (DALYs). They are: Health service priority can be decided by the aid of DALYs Aiding in setting health research priorities Help in identifying disadvantaged group and targeting health interventions Providing a comparable measure of output for intervention/program evaluation and planning. Estimating summary measures of health requires a wide variety of data including: population counts; incidence and mortality rates; life expectancies; cause-specific and observed survival; distributions, durations, and preference scores across a multitude of health states; and risk factor data to estimate population attributable fractions. Disaggregating by age group and sex further explodes the quantity of data5. Quality adjusted life years (QALYs) The concept of Quality Adjusted Life Years (QALYs) was first introduced by Herbert Klarman and colleagues in 1968 where they studied chronic renal failure. Although they did not use the term quality adjusted life years, the concept was identical where they noted that the quality of life of patients with a kidney transplant was better (estimated that it was 25%) than that with dialysis. The cost per life year gained by different treatment option was calculated with and without this quality adjustment6. QALYs is the comprehensive measure of health outcome because it can simultaneously capture gains from reduced morbidity (quality gains) and reduced mortality (quantity gains) and combine these into a single measure. This combination is based on the relative desirability of different outcomes. We know that individuals move through health states over time and that each health state has a value attached to it. Health is defined as the value-weighted time – life - years weighted by their
Malaysian Journal of Public Health Medicine 2011, Vol. 11(2): 27-31 quality—accumulated over horizon to yield QALYs7.
the
relevant
time
In QALYs, premature death is combined with morbidity by attaching a weight to each health state such that value 0 represents death, while value 1 represents full health. The number of QALYs for a health profile is found by multiplying the health related quality of life (HRQoL) weight of the health state, with the duration of the health state2. For example, a person who gets some disability at the age of 10, lives with that condition for 35 years, and suffers premature death at the age of 45. If the life expectancy is 60 years, and the health related quality of life weight associated with that condition is 0.75, the (undiscounted) lifetime QALYs of this person are = 1.0 (value of full health) x 10 (years of living in full health) + 0.75 (Health related quality of life weights of that condition) x 35 (years living in that condition) = 36.25. The QALY loss would be =1.0 (value of full health) x 60 (life expectancy) – 36.25 = 23.75. So how actually this 0.75 come about? It is called health related quality of life weights or disability weights (in this case health related quality of life weights!). According to a guidebook on preference measurement in economic analysis by VA Palo Alto, Health Economics Resource Center, these preference weights reflect the desirability or preference for that health state as estimated directly from the study subjects (direct measurement) or indirectly using a variety of measurement systems (indirect measurement). This preference weight is also known as the utility weight8. Apart from this preference weights, two information are needed to estimate QALYs for a group or population which are descriptions of the various health states experienced during these lifetimes and the duration of these health states8. There are several methods to elicit this so called health related quality of life weights5. It can be divided into two broad categories which are the direct method and indirect method. In direct method individuals are asked to rate the
desirability of various health states. Individuals rank their preferences, making trade-offs between health states and alternatives. Individuals make judgments based on their own relative values for the various domains or characteristics of the health state experienced or described. It is the preferred method if it is difficult to describe the health state or when the investigator is uncertain whether all domains that are important to the study are represented in an alternative method. This direct method also has its disadvantage where it may not distinguish health-related quality of life from other factors, it may not always yield different scores for changes in health that are regarded as clinically significant, higher rates of missing data, complex to administer, burdening study participants and site staff. Standard Gamble, Time Trade Off are the direct method. In the standard gamble (SG) approach5, respondents are asked to choose between the certainty of an intermediate health state, and the uncertainty of a treatment with two possible outcomes, where one of the outcomes is more attractive than the certain outcome, and the other is less attractive (e.g. death). The inclusion of uncertainty makes SG more consistent with standard economic utility theory than the other methods, but it is also probably the cognitively most demanding of the techniques for respondents to answer. In the time trade-off (TTO) methodology people are asked to choose between two certain alternatives involving trade-off between quantity and quality of life. For example, they may be asked how many years of perfect health they would consider equally good as living 10 years with psoriasis. TTO questions are the most common technique for elicitation of HRQoL weights in QALYs, although VAS and standard gamble are also sometimes used. The simplest approach is the so-called Visual Analogue Scale (VAS), where respondents are asked to rate health states on a scale. This scale often has the range from zero to one, where zero is the worst possible outcome (assumed to be death), and one is the best possible (perfect health). VAS is a simple method, but has been strongly criticized for not being choice based, in other words, that it does not involve a trade-off between quantity for quality of years. It is not recommended to use alone in economic evaluation because the method does not give the respondent a choice between two alternatives, and therefore, does not reflect the
Malaysian Journal of Public Health Medicine 2011, Vol. 11(2): 27-31 strength of preference necessary for economic evaluation6.
lost by virtue of being in states other than good health9.
Person trade-off (PTO) is structurally similar to TTO5, but here people are asked to trade off extending the lives of people with full health versus improving the health expectancy of people with some disability from sub-optimal to perfect health. This method was used to elicit disease weights for the DALY methodology from a panel of health experts, while the health related quality of life weights in QALYs are usually found by interviewing lay people and patients.
DALYs measure facilitates comparisons of all types of health outcomes by attaching disease weights where value 0 represents full health and value 1 represents death. Note that these disease weights are the opposite of the HRQoL weights in the QALY. A DALY can therefore be seen as an inverse QALY2. DALYs is calculated based on Years of Life Lost (YLLs) due to premature mortality and healthy Years Lost due to Disability (YLDs) where DALYs = YLLs + YLDs
Indirect methods use multi-attribute health status classification systems to define preference weights for the various health states experienced by subjects in a trial. Using surveys of a sample of the population and direct methods (SG, TTO or VAS transformed to SG), developers of these systems have estimated preference or utility weights for each defined health state in their system. These surveys elicited the sample’s preferences for various individual attributes of health. Preference scores for individual attributes of health have been transformed into a preference weight for each health state or combination of attributes in the system. These preference weights have been integrated into the scoring algorithms in the classification system. Currently, there is no standard agreement on which method is the best to elicit either disease or quality of life weights.
YLL are the mortality component of DALYs and determined by life expectancy at age of death.
Disability adjusted life years (DALYs) The DALY concept was introduced by the World Bank in the World Health Report1. It is primarily a measure of disease burden. However its use in cost effectiveness study is relatively common. A background study by World Bank and World Health Organization on the global burden of disease started to define the concept of DALYs whereby it combines; (a) Losses from premature death, which is defined as the difference between actual age at death and life expectancy at that age in a low-mortality population, and (b) Loss of healthy life resulting from disability1. Two important propositions are underlying the DALY concept, namely (i) that the burden calculated for like health outcomes should be the same, and (ii) that the non-health characteristics that should be considered should be restricted to age and sex7. DALYs extends the concept of potential years of life lost due to premature death (YLL) to include equivalent years of healthy life
YLL = N*L, Where; N is the age and sex specific number of deaths and L is the age and sex specific mean life expectancy. Years lost due to disability (YLD) are the disability component of DALYs YLD = I*DW*L, Where; I is the number of incident cases in the reference period DW is the disability weights L is the average duration of disability (in years) World Bank in its world development report 1993 stated that worldwide, 1.36 billion DALYs were lost in 1990 the equivalent of 42 million deaths in newborn children or 80 million death of age 50 1. According to Global Burden of Disease Study by Murray et al, communicable, maternal, perinatal, and nutritional disorders explain 43.9%; noncommunicable causes 40.9%; injuries 15.1%; malignant neoplasms 5.1%; neuropsychiatric conditions 10.5%; and cardiovascular conditions 9.7% of DALYs worldwide10. DALYs therefore can be used as a unit to measure and compare burden of disease for each diseases as well as well as across nations. Conceptually QALYs is almost similar to DALYs. However there are a few important ways that they are different according to Drummond6: 1. Although life expectancy is used in QALYs, it depends on the situation whereby in DALYs calculation the life expectancy is constant 2. The disability weights used in calculating DALYs are not preference based where elicited from patients or general public such as those use in QALYs. In fact the weights used in DALYs are person
Malaysian Journal of Public Health Medicine 2011, Vol. 11(2): 27-31 trade off scores from a panel of healthcare workers who met in Geneva in August 1995. 3. Although both sets of disability weights are on the same scale where death has a score of 0 and full health state has a score of 1, the QALYs weights can take on any value depending upon the health state while the DALYs weights in contrast can only take on one of seven discrete values. That is, in the DALY system there are only seven health states in addition to dead and healthy. 4. In QALYs age weights is not being used while in DALYs age weights is used which gives lower weights to years of the young and the elderly.
2. Robberstad, B. 2005. QALYs vs DALYs vs LYs gained: what are the differences, and what difference do they make for health care priority setting? Norsk Epidemiologi 2005; 15(2): 183-191.
CONCLUSION
5. Flanagan W, Boswell-Purdy J, Le Petit C, Berthelot, JM 2005. Estimating summary measures of health: a structured workbook approach. Population Health Metrics 2005; 3:5.
Population health is a very important element in order to ensure economic and social development of a country. Morbidity and mortality have long been used as the indicator to measure the impact of health care to population health. However, these indicators are inadequate for assessing people who are not ill but have some limited function which affects their everyday life. Therefore various ways introduced to measure a more comprehensive aspect of population health and various agencies proposed their way of measuring population health. This is done either through multiple standardized indicators or single summary measure of population health. Quality adjusted life years (QALYs) outcome measures in economic evaluations of health interventions. Disability adjusted life years (DALYs) was developed as a measure of disease burden as well as measuring population health so that non fatal outcome could be considered apart from mortality in order to do health resources prioritization. These measures of population health can be used in various aspect such as comparing health between populations, monitoring changes within a population, identifying inequalities, prioritize in health planning and development and etc. Although QALYs and DALYs are almost similar in their basic concept but there are few distinct differences which must be paid attention to in order to correctly utilize these measures.
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3. Iburg KM, Kamper-Jørgensen F. Summary measures of population health: an overview. Danish Med Bull. 2002; 49(3): 256-9. 4. Murray, CJL. 1994. Quantifying the burden of disease: the technical basic for disability adjusted life years. Bulletin of the World Health Organization 1994; 72(3): 429-445.
6. Drummond MF, Sculpher MJ, Torrance GW, O’Brien BJ, Stoddart GL. Methods for the economic evaluation of health care programmes. (3rd ed). Oxford University Press: New York, 2005. 7. Weinstein MC, Torrance G, McGuire A. QALYs: the basics. Value in Health 2009; 12: S5-S9. 8. Sinnott PL, Joyce VR, Barnett PG. Preference measurement in economic analysis. Guidebook. VA Palo Alto Health Economics Resource Center: Menlo Park, CA, 2007. 9. Yusoff AF, Kaur G, Omar MA, Mustafa AN. Malaysian burden of disease and injury study 2008. Institute for Public Health, National Institute of Health, Malaysia. 10. Murray CJ, Lopez AD. Global mortality, disability, and the contribution of risk factors: Global Burden of Disease Study. Lancet 1997; 349(9063): 1436-42.
