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To assess the relation between troponin concentration, assay precision, and clinical outcomes in patients with suspected acute coronary syndrome.Cohort study.Tertiary centre in Scotland.2092 consecutive patients admitted with suspected acute coronary syndrome were stratified with a sensitive troponin I assay into three groups (<0.012, 0.012-0.049, and ≥0.050 μg/L) based on the 99th centile for troponin concentration (0.012 μg/L; coefficient of variation 20.8%) and the diagnostic threshold (0.050 μg/L; 7.2%).One year survival without events (recurrent myocardial infarction, death) in patients grouped by troponin concentration.Troponin I concentrations were <0.012 μg/L in 988 patients (47%), 0.012-0.049 μg/L in 352 patients (17%), and ≥0.050 μg/L in 752 patients (36%). Adoption of the 99th centile would increase the number of people receiving a diagnosis of myocardial infarction from 752 to 1104: a relative increase of 47%. At one year, patients with troponin concentrations of 0.012-0.049 μg/L were more likely to be dead or readmitted with recurrent myocardial infarction than those with troponin concentrations <0.012 μg/L (13% v 3%, P<0.001; odds ratio 4.7, 95% confidence interval 2.9 to 7.9). Compared with troponin ≥0.050 μg/L, patients with troponin 0.012-0.049 μg/L had a higher risk profile but were less likely to have a diagnosis of, or be investigated and treated for, acute coronary syndrome.Lowering the diagnostic threshold to the 99th centile and accepting greater assay imprecision would identify more patients with acute coronary syndrome at risk of recurrent myocardial infarction and death but would increase the diagnosis of myocardial infarction by 47%. It remains to be established whether reclassification of these patients and treatment for myocardial infarction would improve outcome.