prothrombin time

PT assesses the time for plasma to clot after addition of tissue factor (thromboplastin) and calcium, activating the extrinsic pathway (Factor VII) and common pathway (Factors X, V, II/prothrombin, I/fibrinogen). Because Factor VII has the shortest half-life among clotting factors, PT is especially sensitive to early liver dysfunction and vitamin K deficiency, both of which impair synthesis of Factors II, VII, IX, and X. PT is prolonged in: warfarin therapy (inhibits vitamin K epoxide reductase, reducing synthesis of vitamin K-dependent factors II, VII, IX, X, and proteins C/S), liver disease (decreased synthesis of clotting factors), vitamin K deficiency (malabsorption, malnutrition, antibiotic use), DIC (consumption of factors), and specific factor deficiencies (e.g., Factor VII deficiency). The International Normalized Ratio (INR) standardizes PT results across different laboratory reagents/thromboplastins, allowing consistent monitoring of warfarin therapy; therapeutic INR is typically 2-3 for most indications (2.5-3.5 for mechanical heart valves). PT/INR should be distinguished from partial thromboplastin time (PTT), which assesses the intrinsic and common pathways (Factors XII, XI, IX, VIII, X, V, II, I) and is used to monitor unfractionated heparin therapy. Mnemonic: PT/warfarin both have 'T' words with fewer letters (extrinsic pathway, shorter), while PTT/heparin correlate with intrinsic pathway. Clinical use includes preoperative coagulation screening, evaluation of unexplained bleeding, liver function assessment (as part of Child-Pugh and MELD scores), and DIC workup (prolonged PT and PTT with low fibrinogen and elevated D-dimer).