Decision limit for troponin I and assay performance

Abstract

Background The redefinition of acute myocardial infarction by the Joint European Society of Cardiology and American College of Cardiology places troponin at the centre of the diagnostic strategy, in addition to lowering the diagnostic cut-off to the 99th centile of a healthy reference population. The required percentage coefficient of variation (%CV) for the assay at this level is 10. Recent publications have examined the utility of the Bayer ACS:180 troponin I assay at a cut-off of 0·1 μg/L to risk-stratify patients with non-ST-elevation myocardial infarction. Methods This study examines the appropriateness of using this assay at this cut-off in individual patients. It also examines the functional sensitivity of the assay and assesses the impact of sample quality on assay performance. Results At the decision limit of 0·1 μg/L, 8% of patients would be assigned to a different risk group on repeat analysis of the same sample on the ACS:180 due to assay imprecision. The functional sensitivity (at inter-assay %CV = 10) of the ACS:180 troponin I assay was determined to be 0·27 μg/L. Nineteen of 4850 routine samples (0·39%) failed duplicate precision checks as a result of poor sample quality; this was usually due to the presence of small fibrin particles. Conclusions Careful attention to sample quality is vital in troponin I measurement. The use of the Bayer assay for risk stratification at a cut-off of 0·1 μg/L can lead to inconsistency of risk assessment in a small but significant proportion of cases.