Prothrombin Time

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Latest Edit: Hector 2014-03-24 (EDT)

See Also Lab Tests

This test is used to assess the adequacy of the extrinsic system and common pathway in the clotting mechanism.[1]


  • The hemostatic and coagulation system consists of a homeostatic balance between factors encouraging the clotting and factors encouraging clot dissolution.
  • The PT measures the clotting ability of factors I (fibrinogen), II (prothrombin), V, VII, and X, which are clotting factors in the extrinsic system and coagulation pathway
  • When these clotting factors exist in deficient quantities, the PT is prolonged

Patient Preparation

  • No fasting required.
  • Factors which can cause increased levels
  • Drugs: Allopurinol, aminosalicylic acid, barbiturates, beta-lactam antibiotics, chloral hydrate, cephalothins, chloramphenicol, chlorpromazine (Thorazine), cholestyramine, cimetidine, clofibrate, colestipol, ethyl alcohol, glucagon, heparin, methyldopa (Aldomet), neomycin, oral anticoagulants, propylthiourcil, quinidine, quinine, salicylates, and sulfonamides
  • Factors which can cause decreased levels
  • A diet high in fat or leafy vegetables
  • Drugs: Anabolic steroids, barbiturates, chloral hydrate, digitalis, diphenhydramine (Benadryl). estrogens, griseofulvin, oral contraceptives, and vitamin K

Clinical Implications

High levels indicate:

Associated Tests


  1. Pagana Kathleen D, Pagana Timothy J (2006) Mosby's Manual of Diagnostic and Laboratory Tests, Mosby.