Routinely collected health administrative data has become an important data source for investigators assessing disease epidemiology. Our aim was to investigate the implications of identifying acute coronary syndrome (ACS) events in New Zealand (NZ) national hospitalisation data using either the first (primary) or subsequent (secondary) codes.
Using national health datasets we identified all NZ hospitalisations (2014 to 2016) for patients ≥20 years with a primary or secondary International Classification of Diseases 10th Revision, Australian Modification (ICD10-AM) ACS code. Outcomes included 1-year all-cause and cause specific mortality, hospitalised non-fatal myocardial infarction, heart failure, stroke, or major bleeding, and a composite comprising these outcomes.
Of 35,646 ACS hospitalisations, 78.5% were primary and 21.5% secondary diagnoses. Compared to primary coding, patients with a secondary diagnosis were older (mean 77 vs 69 years), more likely to be females (48 vs 36%), had more comorbidity, and were less likely to receive coronary angiography or revascularisation. Higher adverse event rates were observed for the secondary diagnosis group including a three-fold higher 1-year mortality (40 vs 13%) and two-fold higher composite adverse outcome (54 vs 26%).The use of primary codes alone, rather than combined primary and secondary codes, resulted in overestimation of coronary angiography and revascularisation rates, and underestimation of the 1-year case fatality (13.1 vs 19.0%) and composite adverse event rate (26 vs 32%).
Patient characteristics and outcomes of ACS events recorded as primary versus secondary codes are very different. These findings have important implications for designing studies utilising ICD10-AM codes.

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