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Objective: To examine the hypothesis that transition from creatine kinase MB subunits (CK-MB) to troponin as a more sensitive biomarker of myocardial necrosis reduced the 1 year mortality of non-ST elevation acute coronary syndrome (ACS) patients., Design: Retrospective population based cohort study performed in seven tertiary care hospitals in Israel. The patient population comprised all non-ST elevation ACS admissions during a 5 year period (1999-2004). CK-MB was the biomarker for the diagnosis of myocardial infarction (MI) at the time of admission in 14 037 patients (group 1), while 11 643 patients were admitted after the individual hospital laboratory switched to troponin (group 2). Incidence of ACS types, in-hospital management and 1 year survival was assessed., Results: Group 2 patients had a higher frequency of non-ST elevation MI diagnosis (27.9% vs 17.7%, p<0.001) and were more likely to undergo coronary catheterisation during hospitalisation (44.5% vs 37.5%, p<0.001). One year mortality in non-ST elevation MI was lower in group 2 compared to group 1 (24.6% vs 28.1%, p = 0.002). Similarly, the 1 year death rate in the unstable angina group decreased in group 2 compared to group 1 (7.7% vs 8.5%, p = 0.04). However, the overall non-ST elevation ACS 1 year mortality rate did not change (12.4% vs 11.9%, p = 0.27). In multivariate Cox proportional hazard analysis the transition from CK-MB to troponin had no significant effect on overall 1 year mortality (hazard ratio 0.95, 95% confidence interval 0.89 to 1.03)., Conclusions: Transition to troponin as a diagnostic marker of MI led to an increase in the incidence of non-ST elevation MI. This transition was not associated with a decrease in the 1 year non-ST elevation ACS mortality rate.