INR is the International Normalized Ratio, and the normal reference value is 0.8-1.5. The World Health Organization has established the following permissible ranges for the use of oral anticoagulants: INR should be 1.5-2.5 before non-hip surgery, 2-3 before hip surgery, 2-3 before deep vein thrombosis, and 2-4 for pulmonary embolism for the prevention of deep vein thrombosis. For prevention of arterial thrombosis, INR should be 3-4, and for heart surgery with prosthetic valves, INR should be 3-4. Especially for oral anticoagulant drugs such as warfarin, INR needs to be monitored every 3-5 days to prevent excessive high or low INR, which may result in failure of anticoagulant effect or excessive anticoagulation and cause bleeding.