NIH Module 7 – Laboratory Testing, Diagnostic Imaging, and Screening Practice Test

PT/INR measures how quickly the blood clots, reflecting the activity of the extrinsic and common coagulation pathways. The prothrombin time (PT) tests the time it takes for plasma to form a clot after adding tissue factor and phospholipids, which mainly depends on factors II, V, VII, and X (and the vitamin K–dependent factors). The International Normalized Ratio (INR) standardizes this result across laboratories, making it possible to monitor patients on warfarin therapy and to gauge bleeding risk from conditions that affect clotting factor production, such as liver disease or vitamin K deficiency. A longer PT/INR means slower clot formation and higher bleeding risk; a normal PT/INR indicates the clotting process is proceeding within the expected range. While severe liver disease can prolong PT/INR due to reduced factor production, PT/INR itself is not a direct measure of liver, kidney function, or infection.