Endometriosis

The Australian Longitudinal Study on Women’s Health

The Australian Longitudinal Study on Women’s Health (ALSWH) is an ongoing large population-based prospective cohort study focusing on women’s health. Detailed information on the study methods have been published elsewhere (Brown et al. 1999; Dobson et al. 2015; Loxton et al. 2015; Rowlands et al. 2021).

The analysis presented in this report is based on data from 2 ALSWH cohorts –14,247 women born in 1973–78 and 17,015 women born in 1989–95.

Survey data

Data for endometriosis in the 1973–78 cohort were available from 8 surveys between 2000 and 2022. Data for endometriosis in the 1989–95 cohort were available from 5 surveys between 2013 and 2019. The specific ALSWH survey questions used to measure endometriosis are included in Table TN1. For both cohorts, the woman’s survey response date was used as the date of her first record of endometriosis.

Administrative health data

Three additional sources of administrative health data were used to identify records of endometriosis. The codes used to identify endometriosis, and the time periods that data were available for each data source, are presented in Table TN2. Deterministic linkage of Medicare Benefits Schedule (MBS) records and Pharmaceutical Benefits Scheme (PBS) records for all ALSWH participants was conducted using their unique Medicare number. Hospital data, managed by individual states and territories, were extracted by the health data linkage units in these jurisdictions and linked to ALSWH data using probabilistic matching based on name, date of birth, address and address history.

For the 1973–78 cohort, hospital data were available for all 8 Australian states and territories, but the time periods for which data were available varied (see Table TN3). Hospital data for the 1989–95 cohort were not available for New South Wales or the Australian Capital Territory. Hospital data for all other jurisdictions covered most of the ALSWH survey period for this cohort (2012–2018).

For the 1973–78 cohort, 13,501 women (95%) consented to the linkage of survey data with their administrative health records. For the 1989–95 cohort, 16,972 (almost 100%) consented to the linkage of survey data with their administrative health records.

Statistical analysis

The overall prevalence of endometriosis for each cohort was calculated by combining survey data and administrative health data. Women who completed the baseline survey and who were successfully linked with MBS, PBS or admitted patient hospital data were used to calculate prevalence.

The cumulative incidence estimates by age were derived using the Kaplan-Meier method; instead of the traditional method of using time to failure, age at endometriosis diagnosis was used. The confidence intervals were derived using the variance estimate given by Greenwood’s formula (Greenwood 1926).

For women born 1973–78, by age 44–49, a total of 1,914 cases of endometriosis were identified, corresponding to a prevalence of 14.2% (95% CI: 13.6–14.8%). Of these cases, 1,030 (7.6%, 95% CI: 7.2–8.1%) were confirmed by surgery, while 884 (6.6%, 95% CI: 6.1–7.0%) were categorized as clinically suspected.

For women born in 1989-95, by age 26–31,1,502 cases of endometriosis were diagnosed, corresponding to a prevalence of 8.8% (95% CI: 8.4–9.3%). Among these cases, 654 (3.8%, 95% CI: 3.5–4.1%) were confirmed by surgery, while 848 (5.0%, 95% CI: 4.7–5.3%) were categorized as clinically suspected. 

1. (a) Endometriosis first assessed at survey 2 for this cohort.
2. (b) Women’s responses to both questions were combined to form 1 estimate.
3. (c) Question not assessed at survey 4 for this cohort.

(a) Refer to Table TN3 for specific dates.

(a) Public hospital data only.

National Hospital Morbidity Database

The National Hospital Morbidity Database (NHMD) is compiled from data supplied by the state and territory health authorities. It is a collection of electronic confidentialised summary records for hospitalisations (also known as separations or episodes of care) in public and private hospitals in Australia.

The NHMD is based on the Admitted Patient Care National Minimum Data Set (APC NMDS). It records information on admitted patient care in hospitals in Australia, and includes demographic, administrative and length-of-stay data, as well as data on the diagnoses of patients, the procedures they underwent in hospital and external causes of injury and poisoning.

The hospital separations data do not include episodes of non-admitted patient care given in outpatient clinics or emergency departments. Patients in these settings may be admitted later, with the care provided to them as admitted patients being included in the NHMD.

For more information on the NHMD, see National Hospitals Data Collection.

The following care types were excluded when undertaking the analysis: 7.3 (newborn – unqualified days only), 9 (organ procurement – posthumous) and 10 (hospital boarder).

A small number of hospitalisations (<10) were reported with a sex of ‘male’ or ‘other’. To preserve confidentiality, data presented in this report have been restricted to hospitalisations with a sex of ‘female’ only.

Principal and additional diagnosis codes

Diagnoses were coded using the ICD-10-AM, with the edition applicable to the relevant data year as follows:

• Eleventh edition: 2021–22, 2020–21 and 2019–20
• Tenth edition: 2018–19 and 2017–18
• Ninth edition: 2015–16 and 2016–17
• Eighth edition: 2013–14 and 2014–15
• Seventh edition: 2011–12 and 2012–13.

Endometriosis-related hospitalisations were selected from the NHMD as follows:

• for all endometriosis-related hospitalisations, with a principal and/or additional diagnosis of endometriosis (ICD-10-AM codes N80.0–N80.9)
• for hospitalisations with a principal diagnosis of endometriosis, with a principal diagnosis of endometriosis (ICD-10-AM codes N80.0–N80.9)

The diagnosis code for endometriosis of the uterus (N80.0) is also used for the condition adenomyosis (a condition in which the cells that normally line the uterus are found in the muscular wall of the uterus). Although people who have adenomyosis often have endometriosis, endometriosis and adenomyosis are different conditions.

It is not possible to distinguish which of the records with the diagnosis ‘endometriosis of the uterus’ relate to adenomyosis and which relate to endometrial lesions on the surface of the uterus (that is, endometriosis). Records with the diagnosis ‘endometriosis of the uterus’ have been included in this report, which may result in an overestimate of endometriosis hospitalisations.

In Endometriosis and other chronic conditions, diagnoses were identified using the ICD-10-AM codes outlined in Table TN4.

Table TN4: Diagnosis codes for other chronic condition hospitalisations

National Non-admitted Patient Emergency Department Care Database

The National Non-admitted Patient Emergency Department Care Database (NNAPEDCD) is compiled from data supplied by the state and territory health authorities. It is a collection of electronic confidentialised summary records for presentations to public hospital emergency departments in Australia.

The NNAPEDCD is based on the Non-admitted Patient Emergency Department Care (NAPEDC). The NNAPEDCD provides information on the care provided (including waiting times for care) for non-admitted patients registered for care in public hospital emergency departments that have:

• purposely designed and equipped area with designated assessment, treatment, and resuscitation areas
• the ability to provide resuscitation, stabilisation, and initial management of all emergencies
• availability of medical staff in the hospital 24 hours a day
• designated emergency department nursing staff 24 hours per day 7 days per week, and a designated emergency department nursing unit manager.

Emergency departments (including ‘accident and emergency’ or ‘urgent care centres’) that do not meet the criteria above are not in scope for the NMDS, but data may have been provided for some of these by some states and territories.

A small number of emergency department presentations (<10) were reported with a sex of ‘male’ or ‘other’. To preserve confidentiality, data presented in this report have been restricted to emergency department presentations with a sex of ‘female’ only.

Diagnoses were coded using Emergency Department ICD-10-AM Principal Diagnosis Short List, with the edition applicable to the relevant data year as follows:

• Eleventh edition: 2021–22, 2020–21 and 2019–20
• Tenth edition: 2018–19.

Prior to 2018–19, diagnoses were recorded using different classification systems between jurisdictions. In this report 2018–19 is the earliest year of data presented.

ED presentation for endometriosis, and other reported diagnoses, were identified as outlined in Table TN5.

Table TN5: Emergency Department analysis diagnosis codes

For more information on the NHMD, see National Hospitals Data Collection. 

Australian Burden of Disease Study

The Australian Burden of Disease Study undertaken by the AIHW provides information on the burden of disease for the Australian population. Burden of disease analysis measures the impact of fatal burden (or years of life lost, YLL) and non-fatal burden (years lived with disability, YLD), with the sum of non-fatal and fatal burden equating the total burden (disability-adjusted life year, DALY).

The 2023 study builds on the AIHW’s previous burden of disease studies and disease monitoring work. It provides Australian-specific estimates for over 200 diseases and injuries, grouped into 17 disease groups, for 2003, 2011, 2015 and 2018.

For more information, see Australian Burden of Disease Study 2023.

Australian Disease Expenditure Database

The AIHW Disease Expenditure Database provides a broad picture of the use of health system resources classified by disease groups and conditions.

It contains estimates of expenditure by the Australian Burden of Disease Study diseases and injuries, age group, and sex for admitted patient, emergency department and outpatient hospital services, out-of-hospital medical services, and prescription pharmaceuticals. Pharmaceutical benefit scheme expenditure includes over and under co-payment prescriptions.

It does not allocate all expenditure on health goods and services by disease – for example, neither administration expenditure nor capital expenditure can be meaningfully attributed to any particular condition due to their nature.

For more information, see Disease expenditure in Australia 2020–21.

Methods

Crude rates

The denominator for rate calculations was the estimated resident population (ERP) values for females as of 31 December for the given year (for example, crude rates for 2021–22 used the December 2021 population), unless otherwise noted.

Age-standardised rates

Age-standardisation is a method of removing the influence of age when comparing populations with different age structures – either different populations at one time or the same population at different times.

Direct age-standardisation was used in this report. The Australian estimated resident population as at 30 June 2001 has been used as the standard population.

First Nations hospitalisations

Rates were calculated using the female Aboriginal and Torres Strait Islander population estimates and projections (series B) based on the 2016 Census (ABS cat. no. 3238.0; ABS 2019), and non-Indigenous population estimates derived by subtracting the Indigenous population estimates from the female Australian Estimated Resident Population (ABS 2022). Financial year populations were calculated as the average of the 30 June population estimates for the two relevant calendar years.

Age-standardised rates are for ages 15 and over due to small numbers of endometriosis-related hospitalisations in younger age groups. Age-standardised rates calculated using 5-year age groups from 15 to 50+. Rates were standardised to the 2001 Australian standard population.

Hospitalisations by socioeconomic area

Socioeconomic areas are classified according to area-based quintiles using the ABS Index of Relative Socio-economic Disadvantage (IRSD). Further information is available on the ABS website.

The population denominator for rates of hospitalisation by socioeconomic area is the estimated resident population (ERP) values for females as of 30 June 2021.

Hospitalisations by remoteness area

Comparisons of regions in this report use the ABS Australian Statistical Geography Standard (ASGS) 2016 Remoteness Structure. In some instances, data for remoteness areas have been combined because of small sample sizes. Further information is available on the ABS website.

The population denominator for rates of hospitalisation by remoteness area is the estimated resident population (ERP) values for females as of 30 June 2021.

Hospitalisations by country of birth

Country of birth is reported based on the Standard Australian Classification of Countries (SACC), using the Major group classification presented as region of birth. Further information is available on the ABS website.

The population denominator for rates of hospitalisation by country of birth is the estimated resident population (ERP) values for females as of 30 June 2021.

Conversion of International dollars to Australian dollars

Estimated costs for endometriosis from Armour and others (2019) were converted from International dollars to 2017 Australian dollars using purchasing power parity (PPP) conversion factors where Int$1 = US$1, as per the original study methods. In 2017, the conversion factor was Int$1 = AUD$1.477553 (The World Bank 2023).