How were risk factors selected in Australian Burden of Disease Study 2015?
Risk factors were included in the Australian Burden of Disease Study 2015 if burden attributable to the risk factor was estimated in global burden of disease studies or previous Australian studies, or is of substantial national health policy interest.
To be included, the risk factor had to be modifiable, meaning that it could be prevented or modified through intervention and have sufficient evidence of a causal association between risk factor exposure and disease.
Estimates of risk factor exposure data needed to be available at the Australian population level or which could be applied to the Australian population. The diseases linked to the risk factor needed to occur in Australia and estimates of the additional risk of developing or dying from the disease for each risk factor was also required.
As a result, the Australian Burden of Disease Study 2015 identified 38 risk factor components or exposures (such as cannabis and cocaine use) that combine into 18 individual risk factors (such as illicit drug use).
What is a ‘linked disease’ and how were these selected?
A ‘linked disease’ is a condition in the Australian Burden of Disease Study 2015 disease list with a known risk factor. In other words, the disease or injury is ‘linked’ to the risk factor. For example, tobacco use is a risk factor for the linked disease lung cancer.
Linked diseases were included if the link was biologically plausible and if there was currently sufficient evidence of a causal link. The linked diseases were spread across 15 disease groups. Some risk factors had only a single linked disease, while others had association with multiple diseases across disease groups.
Further information on the methods used to select linked disease is in the Australian Burden of Disease Study 2015: methods and supplementary material report.
What is attributable burden and how is it calculated?
Attributable burden is the disease burden ascribed to a particular risk factor. It is the reduction in burden that would have occurred if exposure to the risk factor had been avoided or had been reduced to its lowest level. It is estimated by applying a population attributable fraction to the estimated disease burden for that linked disease.
The population attributable fractions (PAF) is the proportion of a particular disease that could have been avoided if the population had never been exposed to a risk factor. The calculation of PAFs requires as inputs the relative risk (the increased risk of developing or dying from the disease if exposed to the risk factor) and the prevalence of exposure to the risk factor in the population. PAFs can also be calculated directly from comprehensive data sources such as registries.
Further information on estimating PAFs and the data and methods used in the Australian Burden of Disease Study 2015 can be found in the Australian Burden of Disease Study 2015: methods and supplementary material report.
What does the ‘percent of linked’ burden (DALY/YLD/YLL) mean?
The ‘percent of linked’ burden refers to the proportion of disease burden in the disease linked to the risk factor which could have been avoided if there was no exposure, or minimal exposure to the risk factor. For example, 75% of the lung cancer burden was attributable to tobacco use; that is, this amount of lung cancer burden could have been avoided in Australia if there was no exposure to tobacco.
Why can’t risk factor estimates be added together?
Risk factors in the Australian Burden of Disease Study 2015 were analysed independently. As such, it is not possible to add or combine the separate estimates for different risk factors without further analysis, due to complex pathways and interactions between them. For example, if the burden of diabetes attributable to a diet high in sweetened beverages and to overweight & obesity was added, the amount of diabetes attributable would be an overestimate. This is because these risk factors are found along the same causal pathway—high intake of sweetened beverages increases the risk of overweight & obesity, which in turn increases the risk of type 2 diabetes.
Further analysis is needed to combine risk factors. This additional analysis was undertaken for all risk factors combined, and all dietary risks in the Australian Burden of Disease Study 2015.
Further information on the methods used to estimate the burden attributable to a combination of risk factors is found in the Australian Burden of Disease Study 2015: methods and supplementary material report.
Why did the attributable burden due the risk factor change over time?
Changes over time may be due to changes in exposure to the risk factor or change in the burden from linked diseases. Changes in burden from linked diseases may be influenced by other risk factors and changes to treatment or health intervention. The other inputs used to calculate the burden attributable to this risk factor (such as relative risk or the size of the association between the risk factor and the linked disease) were the same in each year.
Why were current guidelines for risk factors not used to determine the exposure not associated with increased risk?
The level of exposure to risk factors that was not associated with increased risk of disease (also known as the theoretical minimum risk exposure distribution, or TMRED) are different to the guidelines because they are for different purposes.
Guidelines reflect the levels of risk that are acceptable by weighing up the risks and benefits associated with exposure to a risk factor and the distribution of exposure to the risk factor in the population.
TMRED reflect the level of exposure where there is absolutely no risk of disease. The risk factor is limited in definition to high or low exposure only and is specific for the outcomes listed in the study.
Can Australia aim to prevent all attributable burden?
In this study current exposure is compared against a theoretical minimum risk exposure distribution (TMRED).
The TMRED is defined for each risk factor as the theoretical minimum exposure for which there is no increased risk of the linked disease. The estimates reflect how much burden can be prevented if exposure in the population was at the theoretical minimum. This amount of exposure to the risk factor may not be achievable, feasible or economically viable; for example no overweight & obesity in the Australian population.
Why is attributable burden higher for some risk factors than others?
The attributable burden is a combination of:
- exposure to the risk factor in the population
- the size of the association between the risk factor and the linked disease
- the number of linked diseases and
- the amount of burden caused from each linked disease.
Why is the risk factor list and the linked diseases different to the Australian Burden of Disease Study 2011?
The lists of risk factors and linked diseases changes between successive burden of disease studies as more research evidence becomes available. This study used the most recently available evidence at the time of analysis and was largely based on the methods used in the GBD 2016 and AIHW review of the literature.
Why do some risk factors include past or life time exposure?
For some risk factors (alcohol, tobacco, illicit drug use, unsafe sex and cancer due to occupational exposure) past exposure is modelled from current exposure to take into account the effect of past exposure on current burden, that is, the lag between exposure and long term outcomes. The method used for these estimates have been developed internationally.
Some risk factors have impacts that occur over the life course as exposure is linked to long term outcomes.
Where to get more information on data and methods used in ABDS 2015
More information on the Australian Burden of Disease 2015 study can be found in the following reports:
For further information or for customised data requests please contact the AIHW Burden of Disease team: [email protected]