Patients with venous thrombosis often require oral warfarin maintenance therapy for a period of time after discharge from the hospital: generally for initial thrombosis, oral administration is recommended for 3 to 6 months; for thrombosis that reappears without a trigger (recurrence of the original site or new thrombosis in other sites) it is recommended to extend oral administration or even for life. The main role of warfarin is anticoagulation, which is to prevent the formation or spread of blood clots. The main side effect of this type of drug is bleeding, and some bleeding may be fatal or disabling. So, how do you achieve effective anticoagulation with warfarin while its risk of bleeding is low? Generally in the clinic, blood is drawn to monitor coagulation, so that the international normalized ratio (INR, also called international normalized ratio) is maintained between 2 and 3, with a target value of 2.5. However, there are many items in the coagulation test, how can patients understand the meaning of them? The following is a brief analysis of my unit’s coagulation test list as an example (note: the meaning of the following items is much more than what I have said, what I have said is closely related to the meaning of our disease): The first: prothrombin time (PT), is a common clinical monitoring index of the exogenous coagulation system, but also the main indicator of the impact of oral warfarin. Patients after oral warfarin manifested as a prolongation of the value of PT, the specific extension of how much depends on the amount of warfarin taken by the patient to decide, the greater the amount taken, the higher the value is generally, or even beyond the detection range. The second item: normal control (NP): is the control sample of PT, generally the value of PT measurement exceeds the value of NP for more than 3 seconds is considered meaningful. The third item: International Normalized Ratio (INR): is an indicator calculated according to a certain formula based on the measured value of PT, and is the preferred indicator recommended by the World Health Organization for monitoring oral anticoagulants such as warfarin. Why is it necessary to calculate the INR when there is PT? Because the reagents used by laboratories in each hospital to determine PT cannot be identical, so laboratories are required to use prothrombin reagents labeled with the International Sensitivity Index (ISI) and then calculate the INR based on the ratio of PT (PT/NP) and the ISI value, so that the PT measured in different hospitals and laboratories are comparable.