Evaluation of Clinical SYNTAX Score in Predicting New-Onset Atrial Fibrillation and Long-Term Prognosis in Patients Hospitalized with Non-ST Segment Elevation Myocardial Infarction

New-onset atrial fibrillation (NOAF) is a frequent complication among patients with non-ST-elevation myocardial infarction (NSTEMI) and is associated with adverse outcomes, making early identification of predictors essential for risk stratification and management. This retrospective cohort study evaluated the prognostic value of the Clinical SYNTAX score (CSS) compared with the anatomical SYNTAX score (SS), GRACE, and CHA2DS2-VASc scores in predicting in-hospital NOAF and long-term outcomes among 979 NSTEMI patients undergoing coronary angiography. Predictors of NOAF, all-cause mortality, and major adverse cardiovascular events (MACEs: death, myocardial infarction, stroke) were identified using logistic and Cox regression models, with receiver operating characteristic (ROC) analysis used to compare the discriminative power of CSS against SS, GRACE, and CHA2DS2-VASc over a median follow-up of 86 months (IQR 36–100; range 0.1–144). NOAF developed in 80 patients (8.2%), with age, female gender, left atrial diameter, left ventricular diastolic dysfunction, and CSS (p = 0.014) identified as independent predictors. CSS’s AUC for predicting NOAF (0.724; 95% CI: 0.673–0.775) was significantly higher than SS’s (0.644; 95% CI: 0.588–0.699; DeLong p < 0.001) and comparable to that of the CHA2DS2-VASc and GRACE scores; a CSS threshold ≥ 17.77 predicted NOAF with 75% sensitivity and 65% specificity. In Cox regression analysis, in-hospital NOAF, age, hypertension, reduced ejection fraction (<60%), and CSS independently predicted long-term all-cause mortality, while NOAF, EF < 60%, age, early AF recurrence, hypertension, and CSS independently predicted MACEs; SS was not a significant predictor in either model. In non-overlapping multivariable models, this advantage reflected the contribution of CSS’s clinical components rather than incremental value of the composite score; CSS nonetheless remained an independent predictor of NOAF and long-term outcomes and may serve as a practical single-score tool for risk-guided management.