Methods and classifications

Comparison with previous reports

The methods to create the National (insulin-treated) Diabetes Register (NDR) have changed in recent years. These include:

  • the way data are processed
  • how eligibility for the NDR is determined
  • changes to the method used to calculate the incidence of insulin-treated type 2 diabetes.

The derivation of the register applied these new methods retrospectively across all years. Because of these changes, results presented in this report and based on the NDR 2018 cannot be compared with earlier publications, or with results based on earlier NDR data.

Diabetes type on the NDR

A health practitioner classifies diabetes type at the time of NDSS registration. But misclassification can occur, as the symptoms of type 1 and type 2 diabetes can be similar, particularly in young adults.

Further, changes in the classification of diabetes type in the NDSS data in 2002–2003 might have resulted in people with insulin-treated type 2 diabetes being misclassified as having type 1 diabetes.

So, as part of processing information from the primary data sources to create the NDR, the reported diabetes type is checked against a set of criteria, and revised where necessary.

This algorithm (a method of calculation) assesses and reclassifies diabetes type for some registrants on the NDSS. The reclassified diabetes type has been used to calculate estimates by diabetes type for the NDR 2018.

The algorithm is based on age at diagnosis and the period between diagnosis and first insulin use, because of the correlation with diabetes type. The algorithm has been periodically updated in consultation with the AIHW Diabetes Expert Advisory Group. But with or without the algorithm, there will always be some level of misclassification.

More information on the algorithm is available on request.

Incidence

Incidence is the number of new cases (of an illness or event) occurring in a population during a given period. Incidence can be described as either a whole number or rate relative to the total number of people at risk.

Incidence should not be confused with prevalence, which refers to the total number or proportion of cases (of an illness or event) in a population at a given point in time.

In this report incidence of insulin-treated diabetes is described over the calendar year—that is, the number of new cases from 1 January to 31 December in the year being reported.

Incidence rates

Throughout this report, incidence rates are calculated and presented based on the number of cases per 100,000 population.

For example, the incidence rate of:

  • type 1 diabetes among males is calculated as a rate per 100,000 males in the Australian population
  • insulin use among people with type 2 diabetes is calculated as a rate per 100,000 NDR registrants with type 2 diabetes not previously using insulin.

The denominator population used to calculate the incidence rates for insulin-treated type 2 diabetes has changed in recent years. As a result, the incidence rates presented in this web report cannot be compared with earlier publications or results based on earlier NDR data.

For more information see Type 2 diabetes population.

Estimated resident populations

Population data were used to derive incidence rates of type 1 diabetes. Population data are sourced from the Australian Bureau of Statistics (ABS), and updated when revised or new estimates become available.

All population estimates that the ABS currently produces are based on area of usual residence. These estimated resident populations are derived from the ABS Census of Population and Housing, and adjusted for deaths, births and net migration.

The estimated resident populations used in this report are based on the population estimates for 30 June 2018.

The Aboriginal and Torres Strait Islander population is calculated from the Census. Because of the smaller Indigenous population, it is difficult to measure population changes accurately between Census years using the same methods as for the Australian population. As a result, the ABS has developed experimental estimates and projections based on the 2016 Census.

All calculations of rates for Aboriginal and Torres Strait Islander people use:

  • the estimated resident populations derived from the 2016 Census for 2005–2016
  • the Series B projected Indigenous populations for 2017 and 2018.

Type 2 diabetes population

All registrants with type 2 diabetes with no record of insulin use were used as the denominator population to derive the incidence rates of insulin-treated type 2 diabetes.

The NDR derived data from the NDSS and APEG was used to identify the population with diagnosed type 2 diabetes. As most people with type 2 diabetes registered with APEG are also registered with the NDSS, the combined NDSS and APEG data are referred to in the web report as NDSS data.

The population data included all people with type 2 diabetes who were diagnosed between 2000 and 2018 and were still alive on 31 December of each year of analysis.

The coverage of the Australian population with type 2 diabetes registered with the NDSS is unknown, as not all people with type 2 diabetes need insulin treatment or register with the NDSS. The NDSS might underestimate people with type 2 diabetes who manage their diabetes primarily through diet, exercise, and medication.

The population for each year of the analysis was derived based on the diagnosis date of the registrants recorded on the NDSS and APEG. Although NDSS data are available from 1987, many people who registered with the NDSS in the early years of operation have a missing diagnosis date. Where diagnosis date was missing, registration date was used as a proxy to derive the population.

Due to concerns with the Aboriginal and Torres Strait Islander population derived from the NDSS, incidence data for insulin-treated type 2 diabetes by Indigenous status have been excluded from this report.

The NDSS might underestimate the number of Indigenous registrants because:

  • identifying as being Aboriginal or Torres Strait Islander is voluntary
  • people might access diabetes-related products through other programs
  • the coding of Indigenous status on the NDSS has changed.

For more information, see Indigenous status.

Age-specific rates

Age-specific rates provide information on incidence in an age group relative to the total number of people at risk in the same age group.

All age-specific rates in this report are presented as new cases per 100,000 population.

Rates were not reported if the number of new cases of insulin-treated diabetes was less than 5.

Age-standardised rates

Age-standardisation is a method used to eliminate the effect of differences in population age structures when comparing populations with different age structures, and where age affects the variable being compared.

All incidence rates presented in this report are age-standardised to the 2001 Australian population, and presented as age-standardised new cases per 100,000 population.

Age-standardised rates were not reported if the number of new cases of insulin-treated diabetes was less than 20. Age groups were combined where there were less than 30 cases in any age group to calculate age-standardised rates.

Time to first insulin use

Time to first insulin use is reported for 2012–2018. It was derived from the date of diagnosis and date of first insulin use recorded on the NDR for people with insulin-treated type 2 diabetes.

Many registrants in the early years of operation of the NDSS have a missing date of diagnosis. This affects the derivation of the time to first insulin use for people with insulin-treated type 2 diabetes, as they might have been diagnosed long before starting insulin treatment and registering with the NDSS.

Looking at missing data for diagnosis date from 2000–2018 showed that the percentage with a missing date of diagnosis fell from 27% to 10%, stabilising at about 10%–13% in 2012–2018. So, the analysis for time to first insulin use was restricted to 2012–2018.

Geography

Statistical area level 3

Data presented for smaller geographical areas are based on the area of usual residence, using the statistical area level 3 (SA3) units defined in the Australian Statistical Geographical Standard (ASGS) 2016. SA3 are geographical areas built from whole statistical areas level 2 (SA2).

In general, the SA3s are designed to have populations of 30,000–130,000. There are 358 spatial SA3 regions covering the whole of Australia without gaps or overlaps. SA3 correspondence files are sourced from the ABS.

Due to the small number of new cases of insulin-treated diabetes in some SA3s, incidence rates have been calculated for a combined 5-year reference period (2014–2018).

Data for a SA3 were not reported if the number of new cases of insulin-treated diabetes was less than 5 for an area.

Incidence rates were not reported if the number of new cases of insulin-treated diabetes was less than 20 in the reference period; this affected 18% of SA3s for type 1 diabetes, and 1.8% of SA3s for insulin-treated type 2 diabetes.

Primary Health Network

Primary Health Network (PHN) organisations connect health services across a specific geographic area (a PHN area). The Australian Government Department of Health defines the boundaries.

There are 31 PHN areas that cover the whole of Australia. PHNs replaced the previous 61 Medicare Locals on 1 July 2015. The boundaries align with Local Hospital Network (LHN) boundaries (or equivalent), and take into account population size, LHN alignment, state and territory borders, patient flow, stakeholder input and administrative efficiencies. Not all PHNs directly align with the ABS ASGS structure.

Statistical area level 2 (SA2) data was combined up to the PHN area levels, using concordance files sourced from ABS for the analysis of type 1 diabetes.

PHN correspondence files are sourced from the ABS for the analysis of insulin-treated type 2 diabetes. For this update, statistical information is presented using the 2017 boundaries of the 31 PHNs.

Table 1.1 shows the number of PHNs by state and territory. Three jurisdictions (Tasmania, the Northern Territory, and the Australian Capital Territory) are single PHNs. Their size, particularly in jurisdictions with large Indigenous populations (such as the Northern Territory and Western Australia), might mask important inter-area variation.

Incidence rates for PHNs with less than 20 new cases of insulin-treated diabetes in the reference period were not reported. In 2018, this affected the Northern Territory and Western Queensland PHNs.

Table 1.1: Number of PHNs, by state and territory

State/territory

Number of PHNs

New South Wales

10

Victoria

6

Queensland

7

Western Australia

3

South Australia

2

Tasmania

1

Australian Capital Territory

1

Northern Territory

1

Total

31

Remoteness area

Comparisons of regions in this report use the ABS Australian Statistical Geography Standard (ASGS) 2016 Remoteness Structure, which groups Australian regions into 6 remoteness areas: Major cities, Inner regional, Outer regional, Remote, Very remote and Migratory.

These areas are defined using the Accessibility/Remoteness Index for Australia (ARIA), which is a measure of the remoteness of a location from the services that large towns or cities provide. Accessibility is based on distance to a metropolitan centre.

A higher ARIA score denotes a more remote location. The category Major cities includes Australia’s capital cities, except for Hobart and Darwin, which are classified as Inner regional. Remote and Very remote areas have been combined in this publication, and Migratory is excluded.

Further information on the ASGS is available on the ABS website.

The coverage of the NDSS might be lower in Remote and very remote areas or across states and territories with large remote communities. This might influence estimates on the number of people with insulin-treated diabetes in these areas on the NDR.

The lower coverage might in part be due to the distribution of NDSS access points, which help deliver support services and products to people with diabetes in all states and territories. These access points are more limited in rural Australia, and unavailable in some remote communities, where other programs are sometimes available.

Socioeconomic area

Socioeconomic classifications in this report are based on the ABS Index of Relative Socio-economic Disadvantage (IRSD). Geographic areas are assigned a score based on social and economic characteristics of that area, such as income, educational attainment, public sector housing, unemployment, and jobs in low skill occupations.

A low score means an area has, on average, more low-income families, people with less training, and higher unemployment, and might be considered disadvantaged relative to other areas with higher scores.

High scores reflect a relative lack of disadvantage, rather than advantage, and the IRSD relates to the average disadvantage of all people living in a geographical area. It cannot be presumed to apply to all individuals living in the area.

For this report, the population is divided into 5 socioeconomic groups, with roughly equal populations (each about 20% of the total), based on the level of disadvantage of the statistical local area of their usual residence.

The first group includes the 20% of the population living in areas with the highest levels of relative disadvantage (referred to as Group 1, lowest).

The last group includes the 20% of the population living in areas with the lowest levels of relative disadvantage (referred to as Group 5, highest).

The IRSD values used in this report are based on the 2016 Census. Further information is available on the ABS website.

Indigenous status

The NDR might underestimate the number of Aboriginal and Torres Strait Islander registrants with insulin-treated diabetes. Reasons are as follows:

  • Indigenous status can only be reported for people who registered on the NDSS after 2005, due to changes in the way this variable is coded. Before 2005, data entry of Indigenous status coded all ‘unknown’ or ‘not stated’ responses to the Indigenous status question as ‘non-Indigenous’. In 2005, an extra value was added to indicate ‘inadequate/not stated’ where Indigenous status was not known. As a result, Indigenous status cannot be determined for 98% of people registered on the NDSS before 2005.
  • Identifying as being of Indigenous origin on both data sources of the NDR (NDSS and APEG) is voluntary.
  • Other programs that give Indigenous Australians access to diabetes-related products might result in lower registration rates for the NDSS, and subsequently the NDR, among Aboriginal and Torres Strait Islander people. For example, programs such as Aboriginal Medical Services and the National Aboriginal Community Controlled Health Organisation—provide Indigenous Australians access to free and subsidised products that people with insulin-treated diabetes need. In addition, NDSS access points are not always available in remote areas.

These factors might lead to an underestimation of the incidence of type 1 diabetes among Aboriginal and Torres Strait Islander people.

As a result of the classification changes in 2005 on the NDSS, it is not possible to derive the prevalent type 2 diabetes population for Aboriginal and Torres Strait Islander people. So the rates of insulin use among Indigenous type 2 diabetes have not been reported. The reporting of insulin-treated type 2 diabetes in Aboriginal and Torres Strait Islander people is an important data gap that requires future data development work.