Abstract: AIM:To explore the relationship between plasma concentrations of myeloperoxidase (MPO) and onset and progress of acute coronary syndrome (ACS) and the clinical value of MPO in the early identification of ACS. METHODS: MPO concentrations were measured by enzyme-linked immunosorbent assays (ELISA). RESULTS: Patients with ACS had significantly higher MPO concentrations than patients with stable angina pectoris (SAP) and control groups (P<0.05). Significant MPO differences were found between SAP patients and control groups (P<0.05). A positive correlation was observed between MPO and neutrophils as well as creatine kinase isoenzyme (CK-MB). MPO did not demonstrate a correlation with age, highly sensitive C-reactive protein (hs-CRP), white blood cell (WBC) count, total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), triglyceride (TG), aspartate aminotransferase (AST), fasting blood glucose (FBG), lactate dehydrogenase (LDH), red blood cell (RBC) count, and platelet count (PC). Forty-one patients in ACS group and 37 patients in the non-ACS group were diagnosed according to clinical manifestations and coronary angiography. The best cut-off point for MPO was identified as ≥212.59 μg/L using the ROC curve (A=0.927, P=0.000), revealing 39 patients with positive MPO in ACS when MPO was ≥212.59 μg/L and two patients with negative MPO in non-ACS group when MPO was <212.59 μg/L. Sensitivity, specificity and total consistent rate of MPO were, respectively, 95%, 86% and 91%. The false negative rate (the rate of misdiagnosis) was 5% and the false positive rate (misdiagnosis rate) was 14%. Positive and negative predictive values were 89% and 94%, respectively. Kappa value (0.819, P=0.000) showed that the two diagnostic methods were in good agreement. Logistic regression identified significant differences in MPO, LDL-C and CK-MB between ACS group and non-ACS group. A predictive model of ACS was established using these variables and the total correct rate of MPO in predicting ACS reached 94.9%. CONCLUSION: MPO is useful in the early identification of ACS.