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Home :: Prothrombin time

Prothrombin Time (PT)

Prothrombin time (PT) measures the time required for a fibrin clot to form in a citrated plasma sample after addition of calcium ions and tissue thromboplastin (factor III).

Purpose

  • To provide an overall evaluation of extrinsic coagulation factors V, VII, and X and of prothrombin and fibrinogen.
  • To monitor response to oral anticoagulant therapy

Patient preparation

  • Explain to the patient that this test is used to determine if the blood clots normally.
  • Check the patient's history for use of medications that may affect test results, including antihistamines, chloral hydrate, corticosteroids, digitalis glycosides, diuretics, glutethimide, griseofulvin, progestin-estrogen combinations, pyrazinamide, vitamin K, and xanthines. Notify the laboratory if such drugs have been used.
  • Tell the patient that a blood sample will be taken. Explain who will per­form the venipuncture and when.
  • Reassure him that drawing a blood sample will take less than 3 minutes.
  • Explain that he may feel slight discomfort from the tourniquet pressure and the needle puncture.
  • When appropriate, explain that this test is used to monitor the effects of oral anticoagulants; the test will be performed daily when therapy begins and will be repeated at longer intervals when medication levels stabilize.
  • Inform the patient that food or fluids need not be restricted before the test.

Procedure and posttest care

  • Perform a venipuncture, and collect the sample in a 7-ml blue-top tube.
  • If a hematoma develops at the venipuncture site, apply warm soaks.
Precautions
  • Completely fill the collection tube, and invert it gently several times to mix the sample and the anticoagulant thoroughly. If the tube is not filled to the correct volume, an excess of citrate appears in the sample.
  • To prevent hemolysis, avoid excessive probing during venipuncture and handle the sample gently.
  • Promptly send the sample on ice to the laboratory

Reference values

Normally, PT values range from 11 to 13 seconds. Values vary, however, depending on the source of tissue thromboplastin and the type of sensing devices used to measure clot formation. In a patient receiving oral anticoagulants, PT is usually maintained between 2 and 2.5 times the normal control value

Abnormal findings

Prolonged PT may indicate deficiencies in fibrinogen; prothrombin; factors V, VII, or X (specific assays can pinpoint such deficiencies); or vitamin K as well as hepatic disease. It may also result from ongoing oral anticoagulant therapy. Prolonged PT that exceeds two and one-halftimes the control value is commonly associated with abnormal bleeding

Interfering factors

  • Failure to fill the collection tube completely (possible false-high)
  • Failure to adequately mix the sample and the anticoagulant, or to send the sample to the laboratory promptly.
  • Hemolysis due to rough handling of the sample.
  • Salicylates, more than 1 g/day (increase)
  • Fibrin or fibrin split products in the sample or plasma fibrinogen levels < 100 mg/ud1 (possible prolonged PT).
  • Antihistamines, chloral hydrate, corticosteroids, digitalis glycosides, diurctics, glutethimide, griseofulvin, progestin-estrogen combinations, pyrazinamide, vitamin K, and xanthines, such as caffeine and theophylline (possible decrease).
  • Corticotropin, anabolic steroids, cholestyramine resin, heparin I. V. (within 5 hours of sample collection), indomethacin, mefenamic acid, paraaminosalicylic acid, methimazole, oxyrhcnbutazone, phenylbutazone, phenytoin, Propylthiouracil, quinidine, quinlnc, thyroid hormones, vitamin A, or alcohol in excess (possible increase).
  • Antibiotics, barbiturates, hydroxytine, sulfonarnides, mineral oil, or clofibrate (possible increase or decrease).

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