What are the patterns of multimorbidity?

In this section, patterns of multimorbidity are explored by identifying pairs of chronic conditions that commonly occur together among people aged 45 and over.

Identifying conditions that commonly co-occur can inform the development of treatment guidelines and pathways that consider these common patterns of multimorbidity.

Chronic conditions that are highly prevalent in a population may be expected to commonly occur together, as multimorbidities, among people in that population. Of the 10 chronic conditions examined in this analysis, the most common chronic conditions among people aged 45 and over in 2017–18 were:

  • arthritis, 34% of people
  • back problems, 26% of people
  • mental and behavioural conditions, 22% of people (Figure 4.1; Data table 4.1).

The 3 most commonly co-occurring condition pairs among people aged 45 and over were:

  • back problems with arthritis. These 2 conditions were estimated to occur together in nearly 1.2 million people aged 45 and over (12%)
  • arthritis with mental and behavioural conditions, affecting just under 1 million people (10%)
  • back problems with mental and behavioural conditions, affecting over 800,000 people (8.7%).

Figure 4.1 shows the individual and co-prevalence of each of the conditions examined in this analysis. The conditions are listed along the row and column headings. Each circle represents the prevalence of the conditions listed in the corresponding row and column headings. The size of the circle is proportional to the prevalence. Where the row and column headings are the same (for example where both are ‘Arthritis’ in the top left hand corner of the figure), the dark blue circle where they meet represents the prevalence of the individual chronic condition in the population (for example, 34.1% of people aged 45 and over had arthritis).

Where the row and column headings are different (for example, where the row heading is ‘Asthma’ and the column heading is ‘Arthritis’), the light blue circle where they meet represents the prevalence of the co-occurrence of the 2 conditions (for example, 6.1% of people aged 45 and over had arthritis and asthma).

Figure 4.1: Prevalence of single and co-occurring self-reported chronic conditions among people aged 45 and over, 2017–18

This figure shows the prevalence of each of the conditions and their co-occurrence in people aged 45 and over. 

In addition to identifying patterns of commonly co-occurring conditions, it is possible to explore other patterns among conditions that occur together by testing the strength of the association between them (Box 8). Stronger associations between conditions may be anticipated where the 2 conditions affect the same body system, share common risk factors, or where one condition may itself be a risk factor for another condition (see What are the risk factors for multimorbidity? for more detail).

Box 8: Testing for association between 2 chronic conditions

Any 2 conditions may occur together in the same person by chance. The expected prevalence of co-occurrence is the prevalence that we would expect if the 2 conditions occurred together purely by chance.

The ratio of the observed (actual) prevalence to the expected prevalence is a measure of how much more often the 2 conditions occurred together among individuals in the population, than would have been expected purely by chance. It can be considered a measure of the strength of association between 2 conditions (Norén et al. 2013). If the ratio is statistically significantly greater than 1, then the 2 conditions are estimated to co-occur more often than would be expected by chance alone (van den Akker et al. 2001). This may indicate an association between the 2 conditions. A higher ratio may indicate a stronger association between the 2 conditions.

See Identifying patterns of multimorbidity in the Technical notes for detail on how this analysis was performed.

After adjusting for age, a statistically significant association was found in over half of the condition pairs tested, that is, they were estimated to occur together in individuals more often than would be expected by chance alone (Figure 4.2; Data table 4.2). The 3 most strongly associated combinations of 2 chronic conditions among people aged 45 and over were:

  • Asthma with COPD. While asthma and COPD are estimated to occur together in only 2.1% of people aged 45 and over (Figure 4.2), this was 3.5 times as high as the expected prevalence. It is known that longstanding asthma is a risk factor for developing COPD later in life, and that both asthma and COPD can occur together (National Asthma Council Australia 2019).
  • Diabetes with chronic kidney disease. Nearly 0.7% of people aged 45 and over are estimated to have both diabetes and chronic kidney disease, based on self-reported information. This was 2.5 times as high as expected. Diabetes is a common cause of chronic kidney disease with about 1 in 3 people with diabetes developing chronic kidney disease (Kidney Health Australia 2019).
  • Selected cardiovascular diseases with chronic kidney disease. Cardiovascular disease occurred with chronic kidney disease in an estimated 0.7% of people aged 45 and over, this was 2.3 times as high as expected. A complex interplay between the effects of chronic kidney disease and cardiovascular disease means that each condition is a risk factor for the other (Kidney Health Australia 2019).

A network of the co-occurrence of the conditions is shown in Figure 4.2. The size of the circles is proportional to the estimated prevalence of the condition shown. The width of the line connecting 2 circles represents the strength of association between the 2 conditions at each end of the line. Use the filter bar to show only conditions that are more strongly associated.

Figure 4.2: Prevalence of self-reported chronic conditions, their co-occurrence and strength of association, among people aged 45 and over, 2017–18

This figure shows the prevalence of each of the conditions in people aged 45 and over, and the strength of association between significantly associated pairs of conditions.

As with estimates of the prevalence of individual chronic conditions and of multimorbidity, it is important to note that estimates of the co-occurrence of the conditions, and of the strength of the association between them, will be influenced by the self-reported nature of the data. For more information see Factors to consider when interpreting results in the Technical notes.

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