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Self-reported data from the Australian Bureau of Statistics (ABS) 2011–12 Australian Health Survey (AHS) (National Health Survey component) provides the opportunity to look at the prevalence of chronic disease comorbidity among the Australian population. Comorbidity data are presented for the following 8 chronic diseases:
These chronic diseases were selected for reporting because they are common, pose significant health problems, have been the focus of recent AIHW surveillance efforts, and action can be taken to prevent their occurrence. Chronic kidney disease is not included because of the identified low numbers using self-reported data. See data limitations below.
Nearly 5 in 10 Australians (46%) have at least 1 of the 8 selected chronic diseases.
Two in 10 Australians (20%) have 2 or more of the 8 selected chronic diseases.
Comorbidity becomes more common as people get older. Nearly 4 in 10 Australians (39%) aged 45 and over have at least 2 of the 8 selected chronic diseases.
Comorbidity refers to any two or more diseases that occur in one person at the same time. This may occur simply by chance; however, more often than not, diseases occur together because there are some associations between them. Diseases may share risk factors (for example smoking is a risk factor for many diseases), or one disease may be a risk factor in itself for another. For example, diabetes is a well-known risk factor for CVD, possibly due to diabetes increasing atherosclerosis (thickening of blood vessel walls with plaque deposits), which is the underlying cause of most CVD in Australia .
Ageing is a factor that has a particularly strong association with comorbidity. This is because older people are more vulnerable to the onset of many diseases, and with an increased life expectancy in Australia, comes a greater opportunity for multiple conditions to arise .
Reporting on comorbidity is important to inform the planning for preventive, management and treatment services, and in the allocation of health care funding.
Comorbidities often indicate the presence of more severe disease. Therefore, comorbidity is associated with poorer prognosis, a greater frequency of visits to GPs and specialists, more hospital admissions and longer stays in hospital. Together these result in higher health care costs .
In addition, clinical treatment for people with comorbidities is more complex and time consuming than for people with a single condition. Multiple medications and therapies are often needed, which requires extra attention from clinicians to reduce the risk of treatment conflicts such as dangerous drug interactions .
Some combinations of comorbidity have particularly high impacts on the complexity of clinical management and health service use. For example:
Most individuals aged 0–44 with any of the 8 listed chronic diseases do not experience comorbidity (Figure 1). The exceptions are arthritis, cancer and COPD where additional diseases are common.
Source: ABS Australian Health Survey 2011–12 (National Health Survey component) (see source data).
People aged 45 and over are more likely than those aged 0–44 to have comorbidities across all 8 chronic diseases. For people with asthma, COPD, diabetes and cancer, the majority have 2 or more additional diseases.
When interpreting self-reported data, it is important to recognise that because we rely on respondents providing accurate information, the outputs may not be a true reflection of the situation. For example, self-reported data for chronic kidney disease substantially underestimate the prevalence when compared to biomedical testing: 1% according to self-reported data versus 10% according to measured results . People also underestimate their weight yet overestimate their height, which are used to calculate body mass index for the assessment of overweight and obesity . Measured data are, therefore, more reliable in such instances.
While the individual prevalence estimates for each disease are from the best available data sources, using the self-reported data from the 2011–12 AHS enables us to look at the association between chronic diseases across the Australian population, which is not possible using separate data sources. The limitations with the data, however, must be recognised.
It is also important to note that information on comorbidities does not tell us about the impact of comorbidities, as this depends on the specific disease combinations and the severity of the individual diseases.