2.1The study population
The design, details, and primary results of the study have been reported previously . To be brief, the purpose of the study is to evaluate different biomarkers’ prognostic value of adverse cardiac events in patients with CAD. From 2011 to 2015, a total of 4078 patients who underwent coronary angiography examination diagnosed as stable angina pectoris (SAP) or ACS at our hospital were included in the study; subjects were required to be 18 years or older and the result of angiography suggested at least one major coronary artery stenosis ≥ 50%. Patients were excluded if they had severe heart failure, atrial fibrillation, aortic dissection, active infective disease, history of malignancy, end stage of renal disease, as well as those in a deep coma. In the current study, patients were also excluded if their blood sample or detailed data were not available. Finally, a total of 3641 patients were included in the present study (Fig. 1).
2.2 Data collection
Baseline data on demographic characteristics, lifestyle risk factors, cardiac history, and medications were collected at the time of enrollment. CAD were classified as SAP or ACS based on the clinical symptoms and the imaging data. Blood samples were collected in the early morning. On the basis of protocol, the blood were obtained by the EDTA-anticoagulated plastic tubes. All the blood samples were centrifuged at 1000 g for 10 min and serum samples were stored at 80℃. Routine laboratory determinations (blood glucose, lipids) were measured using commercial reagents following standard procedures. Patients were considered to be hypertension with BP > 140/90 mmHg or under anti-hypertensive medication. Hyperlipidemia was defined as known but untreated dyslipidemia or current treatment with lipid-lowering medications. Diabetes mellitus (DM) was defined as the presence of symptoms of diabetes and a resting plasma glucose concentration ≥ 200 mg/dL, a fasting plasma glucose concentration ≥ 126 mg/dL, a 2-h plasma glucose concentration ≥ 200 mg/dL in a 75 g oral glucose tolerance test, or taking hypoglycemic agent or other medications for DM. Current smoking was defined smoking if they reported any tobacco use in the last 30 days.
2.3 Plasma ST2 detection
Blood samples were collected within 24 h of hospital admission after at least 8 h of fasting. The ST2 levels were determined in serum in single measurements by using a quantitative sandwich monoclonal enzyme-linked immunosorbent assay (Presage ST2 Assay, Critical Diagnostics, Inc., San Diego, California). A standard curve was constructed. Analysts were blinded to patients’ characteristics and endpoints of the study participants.
2.4 Outcome assessment
Patients were followed up until May 2020 or until the occurrence of cardiovascular events. All participants were followed up by analyses of clinical materials and telephone contact semiannually. The primary endpoint was MACEs, the second endpoint was all-cause death. MACEs was defined as cardiac death, myocardial infarction, unstable angina and unplanned revascularization. All deaths were considered cardiac unless a definitive non cardiac cause was established. Unstable angina pectoris was defined as new or accelerating symptoms of myocardial ischemia accompanied by new ischemic ST-T changes. Myocardial infarction was defined as the rise of cardiac biomarkers with evidence of myocardial ischemia. Unplanned revascularization was diagnosed if the patient underwent percutaneous coronary intervention (PCI) or coronary artery bypass grafting (CABG) with evidence of myocardial ischemia. We obtained follow-up for all patients until the primary outcome or date of censoring. All-cause death was defined as death from any cause. The follow-up time was calculated from the date of cardiac event onset to the date of event occurrence or the date of the last follow-up. Written informed content was obtained from all study participants, and the study was approved by the ethics committee of Chinses PLA General Hospital.
2.5 Statistical analysis
Patients were divided into two groups according to the median level of ST2. Differences in baseline characteristics between the two groups were evaluated by chi-square tests (categorical variables), analysis of variance as appropriate. Variables with a normal distribution are presented as mean ± SD, whereas in case of non-normality the medians are presented. Categorical data are presented as counts or percentages. Kaplan-Meier curves was used to estimate the cumulative incidence risks of outcomes across baseline ST2 levels and compared by log-rank tests. Cox proportional hazards models were used to evaluate the association of baseline ST2 levels with the study endpoints. The results are presented as the hazard ratios (HRs) and 95% confidence intervals (CIs) according to levels of ST2. We fitted two multivariate proportional hazards models. Model 1 was adjusted for clinical variables including age, sex, BMI, current smokers, hypertension, hyperlipidemia, diabetes mellitus, previous myocardial infarction (MI), previous PCI/CABG, TC, TG, HDL-C, LDL-C. Model 2 was based on model 1, with the addition of ST2. The relation of ST2 levels with outcomes is presented with COX proportional hazard models both with ST2 as a continuous variable and with ST2 as a categorical variable. Area under ROC curve (AUC) was used to compare the predictive ability of the parameters of interest. Furthermore, continuous net reclassification index (NRI), and integrated discrimination improvement (IDI) were generated to evaluate any improvement in prognostic prediction when ST2 was added to the established model. SPSS and R 4.0.0 (R Foundation for Statistical Computing) used for descriptive data analysis. All statistical tests were 2-tailed, and p values < 0.05 were considered statistically significant.