Clinical screening tests for blood coagulation that use plasma as samples cannot estimate the participation of platelets, leukocytes, and erythrocytes in blood coagulation system. We developed an assay to evaluate the total coagulation ability of blood and whole blood prothrombin time (WPT) using the principle of prothrombin time with the diluted-tissue factor as a trigger and a newly developed apparatus, STA, viscosity change detection system (electromagnetic clot detection). The activation of platelets by Ca ionophore shortened WPT and increased the expression of CD62P on the platelet surface. WPTs in citrated blood of spontaneous hypertensive rats (SHR) were significantly shorter than those of controls, Wistar Kyoto rats (WKY). Plasma levels of thrombin-antithrombin III (TAT) in SHR were significantly higher than those of WKY. Moreover, WPT and TAT levels were significantly correlated. Based on these results, WPT was found to be useful to estimate the activation of blood coagulation in whole blood.