Value of single troponin values in the emergency department for excluding acute myocardial infarction in Aboriginal and Torres Strait Islander people

Patients with single high sensitivity cardiac troponin I (hs‐cTnI) measurements below specified threshold values are deemed to be at low risk of acute myocardial infarction (AMI).1,2,3,4,5 The accuracy of single hs‐cTnI measurements has not been investigated in Aboriginal and Torres Strait Islander people. The National Heart Foundation of Australia and Cardiac Society of Australia and New Zealand guidelines note that Aboriginal and Torres Strait Islander patients should generally not be treated as being at low risk.6

We evaluated the accuracy of single hs‐cTnI measurements for excluding AMI in Aboriginals and Torres Strait Islanders in a planned analysis of data collected during the observational IMPACT study, which examined the assessment of potential acute coronary syndrome (ACS) in this population. We collected data for adults (18 years or older) who visited the Cairns Hospital emergency department during the 2019 calendar year and were evaluated for ACS. Patients were recruited consecutively on weekdays (8am – 5pm). Further details, including on cultural safety, are included in the . The Far North Queensland Human Research Ethics Committee approved the study (HREC/17/QCH/42–1139).

All participants received standard care, including electrocardiography and hs‐cTnI assessments at presentation and 2–3 hours later (hs‐cTn I assay [Beckman Coulter]; limit of detection, 2.3ng/L), as well as appropriate imaging for coronary artery disease. Thirty days later, research nurses followed up patients by telephone and reviewed medical records for cardiac events, cardiac investigations, and contact with health care providers. The primary endpoint was index type 1 myocardial infarction (T1MI), including non‐ST‐elevation myocardial infarction (NSTEMI), or cardiac death prior to discharge. Secondary outcomes were index AMI (types 1 and 2 MI), and 30‐day major adverse cardiac events (MACE: T1MI, revascularisation, cardiovascular death within 30days of presentation). Test sensitivity and negative predictive value (NPV) were calculated with exact binomial 95% confidence intervals (CIs), for each hs‐cTnI value below the 99th percentile (18ng/L).

A total of 110 Aboriginal and Torres Strait Islander people were included in our study (); 58 were men (53%). Fifteen people (14%) had hs‐cTnI values of 2ng/L or less, none of whom reached the primary or secondary endpoints. Test sensitivity was 100% (95% CI, 93–100%) and the NPV 100% (95% CI, 78–100%) (Box 1). Sensitivity and NPV were also 100% with the cTnI cut‐off recommended by the European Society for Cardiology (4ng/L);8 this cut‐off excluded AMI for 30 people (27%), none of whom reached the primary or secondary endpoints. Diagnostic accuracy was similar for patients who presented to the emergency department and those who had been transferred from other facilities (data not shown).

Our findings support using single low level hs‐cTnI values to exclude AMI in Aboriginal and Torres Strait Islander people; applying either presentation cut‐off value (2 or 4ng/L) enabled early exclusion without missing any cases of AMI or MACE. Large proportions of participants reported risk factors for cardiovascular disease, including smoking (66%), diabetes (48%), hypertension (56%), and family history of coronary artery disease (57%) (Box 2). A low hs‐cTnI value may safely exclude AMI, but Aboriginal and Torres Strait Islander people may benefit from referral to culturally appropriate medical services for cardiac risk factor management.

Our study was limited by the small sample size and its restriction to a single centre. Despite 100% sensitivity and NPV, the lower bounds of the 95% CIs with a 2 ng/L cut‐off indicated a considerable degree of uncertainty. Exclusion algorithms should be implemented in the context of the individual’s clinical history. It is unclear how doing so would have affected diagnostic accuracy.

A single low level hs‐cTnI value can be used for early rule‐out of AMI in Aboriginal and Torres Strait Islander patients. Our study advances understanding of ACS assessment by ensuring that our evidence base reflects the diversity of people in our health care system.

Box 1 – Receiver operating characteristic curve for using single low level high sensitivity cardiac troponin I values in the emergency department to exclude acute myocardial infarction in Aboriginal and Torres Strait Islander people

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Area under the receiver operating characteristic curve = 0.9699.