PT is the plasma prothrombin time, which is the most sensitive and commonly used screening test to detect exogenous coagulation pathways. The normal range is 11-13 seconds, and it is abnormal if it exceeds the normal control by more than 3 seconds. In clinical practice, by detecting PT, it can make a clear judgment on whether the human exogenous coagulation pathway is abnormal. When PT is significantly prolonged, it can be seen in coagulation factor I, coagulation factor II, coagulation factor V, VII, X and other coagulation factor deficiencies, such as severe liver disease, vitamin K deficiency, and disseminated intravascular coagulation, etc., all of which will have significantly prolonged PT. Also, in patients taking oral anticoagulants clinically, such as after oral warfarin, there is also a significant prolongation of PT, and the measurement of PT enables further clinical guidance on the dosage of warfarin. Therefore, PT is a screening test that reflects the exogenous coagulation pathway.