Fibrosis-4 Index as a Marker of Systemic Fibrotic Burden and Its Association with Left Atrial Thrombus in Nonvalvular Paroxysmal Atrial Fibrillation

Background/Objectives: Left atrial thrombus (LAT) is a clinically important finding in nonvalvular atrial fibrillation (AF). The Fibrosis-4 (FIB-4) index reflects systemic fibrotic burden. We investigated the association between FIB-4 and LAT. Methods: This retrospective study included 859 patients with nonvalvular paroxysmal AF undergoing transesophageal echocardiography (TEE). ROC analysis and multivariable logistic regression were performed. Results: Left atrial thrombus (LAT) was detected in 10.2% of patients. Patients with thrombus exhibited significantly higher admission FIB-4 scores compared to those without (1.5 vs. 1.1, p < 0.001). ROC analysis yielded an optimal FIB-4 cut-off of 1.47 (AUC: 0.65, 95% CI: 0.57–0.70, p < 0.001), providing 71.6% sensitivity and 72.0% specificity. After adjusting for CHA2DS2-VASc score, renal function, and left atrial diameter, a FIB-4 > 1.47 remained a strong independent predictor of LAT (OR: 5.200; 95% CI: 3.105–8.708, p < 0.001). However, the addition of FIB-4 to the CHA2DS2-VASc score did not significantly improve discriminatory performance (p = 0.314, DeLong’s test). Spearman’s correlation showed a modest relationship between FIB-4 and CHA2DS2-VASc (r = 0.321). Conclusions: Elevated FIB-4 index values are independently associated with LAT in patients with paroxysmal AF. This simple, noninvasive marker may reflect a systemic fibro-inflammatory milieu that promotes an atrial thrombogenic substrate beyond traditional clinical risk scores.