From the Research
The INR (International Normalized Ratio) is not made by cells but is a laboratory test that measures how long it takes blood to clot. It specifically evaluates the extrinsic pathway of coagulation and is used to monitor patients on warfarin therapy. The test works by measuring prothrombin time (PT), which is then standardized using an International Sensitivity Index to create the INR value. While no cells directly "make" the INR, the test is affected by the function of platelets and clotting factors (particularly factors I, II, V, VII, and X) produced by the liver. These clotting factors are proteins synthesized by hepatocytes (liver cells) and are vitamin K dependent. This is why warfarin, which inhibits vitamin K recycling, affects the INR. Normal INR values range from 0.8-1.2 for those not on anticoagulation therapy, while therapeutic ranges for patients on warfarin typically fall between 2.0-3.0 or 2.5-3.5 depending on the clinical indication. The most recent study 1 from 2022, which compared direct oral anticoagulants (DOACs) versus warfarin in patients with atrial fibrillation and valve replacement or repair, did not directly address the question of what cells make the INR, but it highlights the importance of INR in monitoring warfarin therapy. However, the key point is that the INR is a test result, not a substance produced by cells, and its value is influenced by the liver's production of clotting factors, as noted in various studies 2, 3, 4, 5. In clinical practice, understanding the factors that influence INR is crucial for managing patients on warfarin therapy, but the INR itself is not a product of cellular activity. The focus should be on the liver cells (hepatocytes) that produce the vitamin K-dependent clotting factors, which in turn affect the INR value, as indirectly related to the management of anticoagulation therapy discussed in the provided studies 2, 3, 4, 5, 1.