Background: Cardiac troponin (cTn) testing has reduced the likelihood of erroneous discharge of patients with acute coronary syndrome (ACS) from the emergency department (ED), but doubts remain about optimal clinical use. This study was planned for evaluating the predictive significance of cTn values between the limit of detection of the method and the 99th percentile in ED patients evaluated for suspected ACS. Methods:In this retrospective study all hospital records of patients admitted over a 6-month period to the ED and with at least one cTnI value comprised between the limit of detection (0.01 ng/mL) and the 99th percentile of the assay (0.05 ng/mL) were analyzed.Results: A total of 4,749 patients with cTnI value between 0.01-0.05 ng/mL were identified among 57,879 ED visits throughout the study period. Overall, 2,189 patients (46.1%) were discharged from the ED, 2,529 (53.25%) were admitted to the hospital and 31 (0.65%) died during ED stay. A total number of 289 patients out of 2,189 who were discharged (i.e., 13.2%) had additional ED visits within 30 days. Among these, 6 were diagnosed with ACS, representing 0.27% of patients discharged [negative predictive value (NPV) 0.997; 95% CI, 0.994-0.999] and 2.1% of those with second admission (NPV 0.979; 95% CI,. Only one of the 2,529 patients admitted to the hospital (i.e., 0.04%) developed an ACS during hospital stay. Conclusions:The results of our retrospective study suggest that the suitability of using a contemporary-sensitive cTnI immunoassay assay in the context of an appropriate protocol represents a safe and effective strategy for ruling in and ruling out ACS in patients presenting to the ED.