Abstract : Coumarin-based drugs are widely used to treat thrombotic disorders. The presence of coumarin in plant extracts can be explored to provide alternative treatments to such diseases. This study investigated the effect of Bambusa blumeana leaf extracts using Prothrombin Time (PT) and International Normalized Ratio in vitro using plasma derived from 10 healthy participants (5 male, 5 female). Extracts of varying concentration (25 mg/mL, 50 mg/mL, and 100 mg/mL), a negative control (0 mg/mL) and a positive control (warfarin = 0.016 mg/mL) were used as treatments in the study. Values for PT and INR were recorded by a coagulation analyzer. The minimum values for INR and PT, 0.95 and 11 respectively, were exhibited by the negative control. The maximum values for INR and PT were exhibited by the positive control: 1.58, 17.9 sec respectively. The results of the study show dose-dependent effects among the concentrations of extracts and the PT. Kruskal-Wallis test (p<0.001) confirms a significant difference among extracts (25 mg/mL, 50 mg/mL, 100 mg/mL) and the negative control (0 mg/mL) in both INR and PT. Post-hoc pairwise comparison revealed 100mg/mL as the optimum concentration, p<.001. Mann Whitney U test (p=0.678 for INR and p=0.862 for PT) determined no significant difference with the optimum extract concentration (100 mg/mL) and the positive control (warfarin = 0.016 mg/mL). Future research may investigate the effect of higher concentrations of both the extracts and the positive control and their effect on the optimum concentration of Bambusa blumeana.