What is the impact of missing Indigenous status on mortality estimates? An assessment using record linkage in Western Australia.

BACKGROUND: The analysis aimed to assess the Indigenous status of an increasing number of deaths not coded with a useable Indigenous status from 1997 to 2002 and its impact on reported recent gains in Indigenous mortality.
METHODS: The Indigenous status of WA death records with a missing Indigenous status was determined based upon data linkage to three other data sources (Hospital Morbidity Database System, Mental Health Information System and Midwives Notification System).
RESULTS: Overall, the majority of un-coded cases were assigned an Indigenous status, with 5.9% identified as Indigenous from the M1 series and 7.5% from the M2 series. The significant increase in Indigenous male LE of 5.4 years from 1997 to 2002 decreased to 4.0 and 3.6 years using the M1 and M2 series, respectively, but remained significant. For Indigenous females, the non-significant increase in LE of 1.8 years from 1997 to 2002 decreased to 1.0 and 0.6 years. Furthermore, annual all-cause mortality rates were higher than in the original data for both genders, but the significant decline for males remained.
CONCLUSION: Through data linkage, the increasing proportion of deaths not coded with a useable Indigenous status was shown to impact on Indigenous mortality statistics in Western Australia leading to an overestimate of improvements in life expectancy. Greater attention needs to be given to better identification and recording of Indigenous identifiers if real improvements in health status are to be demonstrated. A system that captures an individual's Indigenous status once and is reflected in all health and administrative data systems needs consideration within Australia.