In July 2015, 29 supplementary codes for chronic conditions (U78–U88), and a new Australian Coding Standard (ACS 0003 Supplementary codes for chronic conditions) were implemented in the Ninth Edition of ICD-10-AM. The introduction of these codes has implications for the monitoring of chronic conditions using hospitalisation data in Australia.

This report aims to: 

• examine the patterns of use of the supplementary codes for chronic conditions in the first 5 years since their introduction (2015–16 to 2019–20) using unlinked hospitalisation data
• examine whether the introduction of the supplementary codes for chronic conditions affected the coding of additional diagnoses for the same chronic conditions
• examine the impact that the inclusion of the supplementary codes for chronic conditions in linked and unlinked data analysis makes on estimates of chronic conditions among the hospitalised population
• assess the consistency of chronic condition coding for an individual over time using linked data.

This report uses data from the National Hospital Morbidity Database and the National Dementia Linked Dataset to evaluate the use of the supplementary codes for the purpose of population health monitoring and provide evidence for including these codes in future analysis and linked data assets.

Key findings

• 1 in 3 hospitalisations in 2019–20 had at least one supplementary code.
• One-third of the conditions for which supplementary codes were introduced in 2015–16, had a marked drop in additional diagnoses in that year.
• Including supplementary codes in analyses increases the capture of some chronic conditions up to 15-fold in unlinked hospitalisations data and 9-fold in available linked hospitalisations data.
• Supplementary codes are only being assigned in a minority of hospitalisations of people who have a prior record of a diagnosis or supplementary code for the chronic condition.