Aim Recent evidence suggests that an undetectable troponin level at emergency department (ED) presentation can rule out the presence of myocardial infarction (MI) in low-risk patients. The aim of this study was to investigate whether an undetectable troponin I (TnI) level at presentation using a contemporary troponin assay can accurately rule out MI at various front-door thrombolysis in myocardial infarction (fTIMI) score cut-offs.

Methods Planned substudy of a prospective observational cohort study of patients presenting to ED with chest pain without ECG evidence of ischaemia who underwent a ‘rule out’ acute coronary syndrome process. Clinical, investigational and outcome data were collected. A contemporary TnI assay (Siemens TnI Ultra) was used. Primary outcome of interest was diagnostic accuracy for MI of undetectable initial TnI at presentation at various fTIMI scores (sensitivity, specificity, positive predictive value and negative predictive value (NPV)).

Results 1076 patients were studied, of whom 156 had a final diagnosis of MI (14.5%). For patients with undetectable TnI and fTIMI scores 0, 0–1, 0–2 and 0–6, sensitivities were 98.7%, 98.1%, 97.4% and 97.4%, respectively, specificities were 22.6%, 41.7%, 53.8% and 69.9%, respectively, and NPV were 99%, 99.2%, 99.2% and 99.4%, respectively. If early presenters (<2 h of symptoms) were excluded, undetectable initial troponin had 100% sensitivity (95% CI 95.2% to 100%) and NPV (95% CI 98.8% to 100%).

Conclusions Using a contemporary TnI assay, undetectable initial TnI has high but not perfect sensitivity and NPV, unless early presenters are excluded.

Trial registration number ACTRN12612000990820.