2. Prothrombin is a protein produced by the liver for clotting of blood. Prothrombin production depends on adequate vitamin K intake and absorption. During the clotting process, prothrombin is converted to thrombin. The prothrombin content of the blood is reduced in patients with liver disease. The PT is one of the four most important screening tests used in diagnostic coagulation studies. It directly measures a potential defect in stage II of the clotting mechanism (extrinsic coagulation system) through analysis of the clotting ability of five plasma coagulation factors (prothrombin, fibrinogen, factor V, factor VII, and factor X). In addition to screening for deficiency of prothrombin, the PT is used to evaluate dysfibrinogenemia, the heparin effect and coumarin effect, liver failure, and vitamin K deficiency.
7. Oral Anticoagulant Therapy :t he se drugs act through the liver to delay coagulation by interfering with the action of the vitamin K–related factors (II, VII, IX, and X), which promote clotting Oral anticoagulant drugs (eg, Coumadin, dicumarol) are commonly prescribed to treat blood clots. These are indirect anticoagulants (compared with heparin, which is a direct anticoagulant). Oral anticoagulants delay vitamin K formation and cause the PT to increase as a result of decreased factors II, VII, IX, and X. The usual procedure is to run a PT test every day when beginning therapy. The anticoagulant dose is adjusted until the therapeutic range is reached. Then, weekly to monthly PT testing continues for the duration of therapy Coumadin takes 48 to 72 hours to cause a measurable change in the PT (3–4 days of drug therapy).
8. Patients with cardiac problems are usually maintained at a PT level 2 to 2.5 times the normal (baseline) values Use of the INR values allows more sensitive control. A reasonable INR target for virtually all thromboembolic problems is 2.0 to 3.0 For treatment of blood clots, the PT is maintained within 2 to 2.5 times the normal range. If the PT drops below this range, treatment may be ineffective, and old clots may expand or new clots may form. Conversely, if the PT rises above 30 seconds, bleeding or hemorrhage may occur.
