International normalized ratio (INR) is a calculation made to standardize prothrombin time. INR is based on the ratio of the patient's prothrombin time and the normal mean prothrombin time. Prothrombin time is a test to learn how fast the blood clots in patients receiving oral anticoagulant medication. This test may be used in patients with prosthetic heart valves, venous thromboembolism, or antiphospholipid syndrome[1][2][3][4][5][6][7][8][9]. A blood sample is collected for this test[6][9].