Compared with pre-linkage notification rate ratios, we found no significant difference in age-adjusted STI and BBV notification rate ratios by Aboriginality in Western Australia following data linkage. Complete and accurate data on Aboriginality are required for the valid estimation of STI and BBV notification rates, and despite investigating the influence of both incomplete data and the inconsistent classification of Aboriginality on Aboriginal to non-Aboriginal notification rate ratios in 2010, variability in the post-linkage estimates was small. These findings support the validity of estimating disease-specific notification rate ratios in 2010 based on the exclusion of cases with unknown Aboriginality, and contrast with an analysis of 2004 data which found that the exclusion of cases with unknown Aboriginality prior to linkage overestimated disease-specific notification rate ratios for some diseases [5].
A review of the findings in 2004 reveals that data linkage identified data on Aboriginality for a similar proportion of notifications with unknown Aboriginality prior to linkage in 2004 (73.5%) and 2010 (73.9%), and that only a small proportion of notifications with unknown Aboriginality prior to data linkage were ever identified as Aboriginal following data linkage in both 2004 (5.8%) and 2010 (4.9%). However, the proportion of STI and BBV notifications with unknown Aboriginality prior to data linkage decreased approximately 60% from 26% in 2004 to 10% in 2010, and there was a large decrease in age-adjusted notification rate ratios by Aboriginality between 2004 and 2010 for all STIs and BBVs apart from hepatitis C. Improved accuracy of identification of Indigenous status in more recent data has also been found in a study of the Western Australian Hospital Morbidity Data Collection [11], suggesting that recent improvements in data quality are not limited to notifiable disease data.
Improved completeness of Indigenous status in the notification data can be attributed to better reporting by medical practitioners and greater awareness of the importance of high levels of data completeness among staff in public health units who receive and manage the data. STI and BBV case notifications with missing data on Indigenous status are followed-up by public health unit staff who use several strategies to improve the completeness of notifications, including contacting the notifying medical practitioner; using their knowledge of the local community, particularly in regional areas where the local community is small; and using information from previous notifications and other administrative health data collections, including the public hospital database ‘The Open Patient Administration System’ (TOPAS).
There is currently no mechanism in the Western Australian Notifiable Infectious Diseases Database to identify whether notification data on Indigenous status were obtained from the original notification or during follow-up, and only a detailed audit of the original paper notification records could retrospectively identify the contribution of follow-up processes to reporting completeness. However, cases which are only laboratory notified (i.e. no notification is received from a medical practitioner) do not include information on Indigenous status, and the development of strategies to establish reporting of Indigenous status for these cases is required. Anecdotal information indicates that follow-up processes conducted to improve the completeness of data on Aboriginality are essential to maintain a high level of identification of Indigenous status in the STI and BBV notification data. Ensuring that the contribution of follow-up processes to reporting completeness are identifiable could enable both the evaluation of strategies developed to improve reporting completeness and the development of processes to maintain the high levels of reporting achieved in 2010.
The completeness of Aboriginality in the notification data varied by disease according to previously reported patterns [5], and the low proportion of syphilis and gonorrhoea notifications with unknown Aboriginality prior to data linkage reflect the use of enhanced surveillance processes for these diseases in Western Australia [20]. STI and BBV rates and rate ratios can reveal changes in endemic and epidemic activity as well as changes in disease detection and control efforts. The largest decline in the estimated age-adjusted rate ratio between 2004 and 2010 was observed for infectious syphilis, which reflects a continuation of the documented decline in notifications among Aboriginal people and an increase in notifications among non-Aboriginal people [21].
In the absence of a gold standard indicator of Indigenous status, the interpretation of these findings depends on assumptions about the accuracy of information on Aboriginality in the linked data and the validity of methods used to define Aboriginality. Data linkage can be used to improve the quality of Indigenous data [22], but has a limited ability to identify the misclassification of Indigenous status in administrative health data. Research suggests that administrative data collections are likely to under-identify Aboriginal people [11], and we found that unknown Indigenous status in STI and BBV notification data is significantly associated with sociodemographic factors and disease, with unknown Aboriginality more likely to occur among notifications identified as non-Aboriginal following data linkage.
Little data are available to allow estimation of the extent of misclassification of Aboriginality in infectious disease notification data, and identification of the most appropriate definition of Aboriginality when multiple inconsistent indicators exist. Due to inconsistency in the coding of, and uncertainty associated with the validity of indicators of Aboriginality in the linked data, two different criteria were used to examine the impact of classification method on the disease-specific standardised rates and rate ratios. Alongside research demonstrating poorer identification of Aboriginal people in administrative health databases and more accurate coding of Indigenous status among non-Aboriginal people [11, 14], our finding of poorer consistency in reporting among notifications ever identified as Aboriginal supports the use of less rigorous criteria to identify notifications among Aboriginal people based on multiple linked records.
Although disease-specific notification rates were higher than pre-linkage estimates when the ever-Aboriginal definition of Aboriginality was used, significant differences in disease rates were found only for Chlamydia, which had the highest proportion of records with unknown Aboriginality prior to linkage. The use of the ever Aboriginal indicator had little effect on the estimated disease-specific notification rate ratios by Aboriginality despite the potential vulnerability of this indicator to overestimation based on the presence of a single misclassification. Given the likely underestimation of Aboriginality in administrative health data, and the similarity between the three notification rate ratio estimates in this analysis, our findings suggest that indicators based on ever Aboriginal and never Aboriginal may provide an appropriate basis for the calculation of STI and BBV notification rate ratios until an indicator with improved validity is available.
A range of factors have been found to influence the collection and recording of indigenous status in health records, including a lack of awareness and training among staff, staff reluctance to ask the question, staff perceptions that Indigenous Australians do not want to disclose their status; a lack of privacy when answering the question; refusal to answer the question; and little validation of data or follow-up of missing information [10]. Despite the development of best practice guidelines to promote the collection of correct and consistent information on Aboriginality [10], information on Indigenous status is not always consistently sought by health service providers or consistently provided by health service consumers, and self-identification may vary in different contexts. Guidelines recommend the need for staff training; mechanisms for quality assurance and validation, including business rules for checking indigenous status against other data items; and the need to ensure consistency between identifications when there are multiple sources of data [10].
Limitations of this analysis include the lack of a gold standard for comparison and the large variation observed in the number of linked records identified for each individual. The presence of inconsistent classification of Aboriginality examined in this analysis provides only a limited indicator of misclassification in the linked data. The validity of notification rate ratios can also be affected by inaccuracies in the estimated population denominators, the failure to notify, and the under-diagnosis of STIs and BBVs, particularly in rural and remote areas where there is poorer access to health services. In addition, summary rate ratios can mask significant variation in age-specific rate ratios, and some estimates were based on small case numbers.
A high level of completeness of data on Indigenous status in STI and BBV notifications is critical for the accurate estimations of disease rates by Aboriginality. Given the sustained poor reporting of Indigenous status among STI and BBV notifications nationally, there is a need to consider the use of additional strategies to improve the completeness of reporting. Strategies could include better utilisation of existing data linkage resources and the development of indicators that can be used to enable more complete and accurate identification of Aboriginality in routine health surveillance analyses.
The routine linkage of Western Australian notification data with the WADLS could be utilised to improve completeness of data on Indigenous status through the internal linkage of notification data from multiple years by individual. Internal data linkage by individual requires few additional resources, although is likely to provide only modest improvements in data completeness compared with external linkage with other data collections. External linkage is currently more resource intensive and unlikely to be cost effective when there is a low proportion of notifications with unknown Aboriginality. However, routine external linkage may be an effective strategy for other notifiable diseases or in other jurisdictions where the completeness of Indigenous status is low.