PT

PT evaluates the extrinsic and common coagulation pathways by measuring the time for plasma to clot after addition of tissue factor (thromboplastin) and calcium. It is primarily sensitive to deficiencies or inhibition of factor VII (extrinsic pathway) as well as factors X, V, II (prothrombin), and fibrinogen (common pathway). Clinically, PT is prolonged in: Vitamin K deficiency (factors II, VII, IX, X depend on vitamin K for synthesis), liver disease (decreased synthesis of clotting factors), Warfarin therapy (inhibits vitamin K epoxide reductase, reducing synthesis of factors II, VII, IX, X), Factor VII deficiency, and DIC (consumption of clotting factors). Because PT reagents vary between labs, the INR standardizes PT results to allow comparison across laboratories, which is essential for monitoring patients on warfarin (goal INR typically 2-3 for most indications, 2.5-3.5 for mechanical heart valves). PT is often ordered alongside PTT (partial thromboplastin time, which assesses the intrinsic pathway factors XII, XI, IX, VIII) to help localize coagulation defects. A mnemonic: PT measures the Extrinsic pathway and is used to monitor Warfarin (both start with vowel-sounding letters: 'play tennis outside' vs 'play table tennis inside' for PTT/intrinsic/heparin). Understanding PT is crucial for diagnosing bleeding disorders, liver function assessment, and anticoagulation monitoring.