Abstract: AIM: To compare the therapeutic efficacy and safety of two initial warfarin doses for atrial fibrillation(AF) and to further study the risk factors of hemorrhage.METHODS: A retrospective study was adopted. The inpatients with AF in recent 2 years were divided into two groups according to their initial warfarin doses: ultra-low dose group(1.25-1.875 mg) and low dose group (2.5-3.0 mg). The efficacy and safety of initial therapy of two groups were compared. Meanwhile, the major risk factors of bleeding were analyzed with logistic regression.RESULTS: There was no significant difference between two groups with respect to the safety (P>0.05).However, the mean time to reach a therapeutic INR was significantly longer in the ultra-low dose group than that of low dose group (P＜0.05). In addition, the proportion of INR below the therapeutic range was much higher and the proportion of INR within the therapeutic range was much lower in ultra-low dose group than that of low dose group (P＜0.001). Elderly (age>65 years), abnormal liver function, bleeding, stroke and amiodarone may increase the risk of hemorrhage significantly (OR>1,P<0.05).CONCLUSION: In the patients with AF, the ultra-low initial dosage has equal safety with regard to hemorrhage but increases the risk of thrombosis.At the same time,monitoring of the patients who combined amiodarone should be strengthened.

Key words: Warfarin, Atrial fibrillation, Initiation dosage, Hemorrhagic risk factors