For the first part of the study, we obtained whole blood samples from 20 healthy volunteers who had not taken any medications (12 males and eight females; aged 29 - 54 years). All blood samples were numbered in order of registration. To confirm the in vitro antithrombotic effects of Rivaroxaban and apixaban at the expected clinical peak drug level, blood samples were prepared with drugs as follows. Rivaroxaban and apixaban were obtained from Toronto Research Chemicals (Toronto, Ontario, Canada). Rivaroxaban and apixaban were each dissolved in 100% dimethylsulfoxide to yield stock solutions with a concentration of 5 mM. Stock solutions were then diluted in blood samples at the time of assay (first 16 samples in the Rivaroxaban group and the rest of four samples in the apixaban group). The target final concentration of Rivaroxaban in this experiment was 800 μM, which corresponds to the expected peak concentration (Cmax-equivalent) of Rivaroxaban in blood after a dose of 15 mg/day [12 (link)]. Similarly, blood samples were spiked with apixaban stock solutions. The final concentration of apixaban was 450 μM, which corresponded to the peak concentration of apixaban in blood after a dose of 10 mg/day clinically [13 (link), 14 (link)]. We immediately evaluated the antithrombotic effects of these spiked blood samples in comparison with thrombus formation in control blood by the T-TAS®.