Abstract Background Left atrial (LA) thrombus in patients (pts) presenting for LA ablation is an absolute contraindication to ablation. Although thrombi can still form despite the use of oral anticoagulation (OAC) in advance of the procedure. Transesophageal echocardiography (TEE) has been considered the gold standard for LAA thrombus detection. Additional cardiac computer tomography (CT) is often performed in pts with persistent atrial fibrillation (AF) or atrial tachycardia (AT). The aim of the study was in routine workflow 1) to identify the incidence of LAA thrombi in TEE and CT, 2) to search for an association in detecting LAA thrombi in the 2 methods and 3) to find out if the LAA flow velocity/ LA volume influences the thrombus detection. Methods Between 05/2018 and 10/2023 patients with AF/AT under OAC which were scheduled for a LA ablation were included. TEE and CT searching for LAA thrombus were carried out at intervals of up to 7 days. LAA flow velocity measured in the TEE. The LA volume which included LAA and excluded the pulmonary veins were obtained from the CT at the end-systole. Results Patients characteristics: 283 pts were included, mean age of 66y (+9y), 182 male, mean CHA2DS2-VASc score 3 [0-7]. All patients, mostly on NOACs (n=265), were orally anticoagulated. Mean LA volume (LA + LAA) was 158 ml (+45ml), LAA flow velocity was reduced (< 40 cm/s) in 119 pts (42%). In the majority of cases (73%), TEE and CT were performed at the same day or with an interval of 1 day. In 16 pts a LAA thrombus was detected using TEE,42 pts had SEC/Sludge but no solid thrombus. An LAA thrombus in the CT was described in 26 pts (10%), 6% couldn't definitively diagnose thrombi due to contrast recesses/flow phenomena in 16 pts. In 8 out of 34 events (23%) TEE and CT agreed on the diagnosis of a thrombus, in 6 cases during same-day examinations. 18 pts: LAA thrombus on CT, not on TEE (mean interval: 2d). 8 pts: LAA thrombus on TEE, not on CT (mean interval: 1.5d). In 71% (200 out of 283) of cases, CT and TEE agreed in ruling out a thrombus (p=0.358). LAA flow velocity and thrombus verification by TEE showed a significant correlation (p < 0.001). No correlation was found between the presence of a thrombus on CT and LA volume (p= 0.964). Discussion In 12% of the patients, all treated with OAC, a thrombus was described in at least one of the examinations or in both. Only in one out of four pts TEE and CT agreed on the diagnosis of a thrombus. It was notable that both examinations excluded thrombi in 70%. LAA flow velocity and thrombus verification in the TEE are significant correlated. LA volume and thrombus notification in CT are not related. Conclusion Routine diagnosis of LAA thrombus remains challenging, with frequent discrepancies between TEE, the gold standard, and CT. TEE results depend on the examining physician, while CT is limited by flow phenomena and scan timing.