BackgroundCritically ill medical patients face a heightened risk of developing venous thromboembolism (VTE). In Thailand, routine thromboprophylaxis is not employed. The incidence of deep vein thrombosis (DVT) in the medical intensive care unit (ICU) has not been elucidated in the Thai population. ObjectivesThe aims were to evaluate the incidence of DVT and identify associated risk factors in critically ill medical patients. MethodsA single-center, prospective cohort study was conducted from 2019 to 2020. Consecutive patients underwent screening for proximal deep vein thrombosis by Duplex ultrasound of both legs. ResultsA total of 200 patients were enrolled, with 115 being male (57%). The mean (standard deviation, SD) age was 66.5 (16.4) years. The mean APACHE ll score was 27 (8). The cumulative incidence of DVT over 5 days was 7% (95% confidence interval [CI] 3.4 to 10.6%). No clinically or radiologically diagnosed pulmonary embolism occurred in patients with DVT. No independent risk factor associated with DVT were identified. Hospital mortality in those with and without DVT was 42.9% and 32.3%, respectively. There was no significant difference in the length of ICU or hospital stay, nor inpatient mortality, between those with and without DVT. ConclusionsWithout thromboprophylaxis, the incidence of DVT in the Thai population remains low. A strategy of screening ultrasound five to seven days after admission to the ICU may be a suitable alternative to anticoagulant prophylaxis in critically ill Thai patients who are without symptoms of venous thromboembolism.