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Introduction ⬇

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.

Oral anticoagulant drugs (e.g., warfarin, dicumarol) are commonly prescribed to treat blood clots. These are indirect anticoagulants (compared with heparin, which is a direct anticoagulant). However, if necessary, heparin is the anticoagulant of choice for initiating treatment because it acts rapidly and also partially lyses the clot.

  1. These 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.

  2. Oral anticoagulants delay vitamin K formation and cause the PT to increase as a result of decreased factors II, VII, IX, and X.

  3. The usual procedure is to run a PT/INR test every day when beginning therapy. The anticoagulant dose is adjusted until the therapeutic range is reached. Then, weekly to monthly PT/INR testing continues for the duration of therapy.

  4. Coumadin takes 48–72 hours to cause a measurable change in the PT/INR (3–4 days of drug therapy).

  1. Patients with cardiac problems are usually maintained at a PT level 2–2.5 times the normal (baseline) values.

  2. Use of the INR values allows more sensitive control. A reasonable INR target for virtually all thromboembolic problems is 2.0–3.0. See Table 2.10 for more specific guidelines.

  3. For treatment of blood clots, the PT is maintained within 2–2.5 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.

Normal Findings ⬆ ⬇

PT: 11.0–13.0 seconds (can vary by laboratory)

Therapeutic levels are at a P/C ratio of 2.0–2.5. Recommended therapeutic ranges are shown in Table 2.10.

P/T ratio (prothrombin time ratio): the observed patient PT divided by the laboratory PT mean normal value

INR (International Normalized Ratio): A comparative rating of PT ratios (representing the observed PT ratio adjusted by the International Reference Thromboplastin)

Normal: 0.8–1.2

Patients on anticoagulants: 2.0–3.0 (<2.0 may indicate inadequate protection from clotting and if >4.0 may indicate blood is clotting too slowly)

ISI (International Sensitivity Index): A comparative rating of thromboplastin (supplied by the manufacturer of the reagent, generally between 1.0 and 2.0) with an international World Health Organization standard.

Clinical Alert

  1. Critical value: If PT is >30.0 seconds, notify the healthcare provider (for patients on anticoagulant).

  2. If PT is excessively prolonged (>30 seconds), vitamin K may be ordered.

  3. Critical value: >20 seconds (for nonanticoagulated persons)

  4. Baseline PT levels should be determined before anticoagulant administration.

  5. Critical value: INR >3.6; notify the healthcare provider (for patients on anticoagulants)

Procedure ⬆ ⬇

  1. Draw a 5-mL venous whole blood sample (by the two-tube technique) into a tube containing a calcium-binding anticoagulant (sodium citrate). The ratio of sodium citrate to blood is critical.

  2. Use blue-topped vacuum tubes that keep prothrombin levels stable at room temperature for 12 hours if left capped (vacuum intact). Label the specimen with the patient’s name, date and time of collection, and test(s) ordered. Place the specimen in a biohazard bag.

Clinical Implications ⬆ ⬇

  1. Conditions that cause increased PT include:

    1. Deficiency of factors II (prothrombin), V, VII, or X

    2. Vitamin K deficiency, newborns of mother with vitamin K deficiency

    3. Hemorrhagic disease of the newborn

    4. Liver disease (e.g., alcoholic hepatitis), liver damage

    5. Current anticoagulant therapy with warfarin

    6. Biliary obstruction

    7. Poor fat absorption (e.g., sprue, celiac disease, chronic diarrhea)

    8. Current anticoagulant therapy with heparin

    9. DIC

    10. Zollinger-Ellison syndrome

    11. Hypofibrinogenemia (factor I deficiency), dysfibrinogenemia

    12. Circulating anticoagulants, lupus anticoagulant

    13. Premature newborns

  2. Conditions that do not affect the PT include:

    • Polycythemia vera

    • Tannin disease

    • Christmas disease (factor IX deficiency)

    • Hemophilia A (factor VIII deficiency)

    • von Willebrand disease

    • Platelet disorders (idiopathic thrombocytopenic purpura)

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain the purpose, procedure, and need for frequent testing. Emphasize the need for regular monitoring through frequent blood testing if long-term therapy is prescribed. Do not refer to anticoagulants as “blood thinners.” One explanation might be, “Your blood will be tested periodically to determine the pro time, which is an indication of how the blood clots.” The anticoagulant dose will be adjusted according to PT results.

  2. Caution against self-medication. Ascertain what drugs the patient has been taking. Many drugs, including over-the-counter medications, alter the effects of anticoagulants and the PT value. Aspirin, acetaminophen, and laxative products should be avoided unless specifically ordered by the healthcare provider.

  3. Instruct the patient never to start or discontinue any drug without the doctor’s permission. This will affect PT values and may also interfere with the healing process.

  4. Counsel regarding diet. Excessive amounts of green, leafy vegetables (e.g., spinach, broccoli) will increase vitamin K levels and could interfere with anticoagulant metabolism. Caution against using razor blades; electrical shavers should be used.

  5. These guidelines also apply to posttest patient care.

Posttest Patient Care

  1. Review test results; report and record findings. Modify the nursing care plan as needed. Counsel the patient regarding abnormal findings; explain the need for possible follow-up testing and treatment.

  2. Avoid intramuscular injections during anticoagulant therapy because hematomas may form at the injection site. As the PT increases to upper limits (more than 30 seconds), assess carefully for bleeding from different areas; this may require neurologic assessment (if cranial bleeding is suspected), lung assessment and auscultation, gastrointestinal and genitourinary assessments, or other assessments as appropriate. Instruct the patient to observe for bleeding from gums, blood in the urine, or other unusual bleeding. Advise that care should be exercised in all activities to avoid unintentional injury.

  3. Alert patients who are being monitored by PT for long-term anticoagulant therapy that they should not take any other drugs unless they have been specifically prescribed.

  4. When unexpected adjustments in anticoagulant doses are required to maintain a stable PT, or when there are erratic changes in PT levels, a drug interaction should be suspected, and further investigation should take place.

  5. Make changes in exercise intensity gradually or not at all. Active sports and contact sports should be avoided because of the potential for injury.

  6. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆

  1. Diet: Ingestion of excessive green, leafy vegetables increases the body’s absorption of vitamin K, which promotes blood clotting.

  2. Alcoholism or excessive alcohol ingestion prolongs PT levels.

  3. Diarrhea and vomiting decrease PT because of dehydration.

  4. Quality of venipuncture: PT can be shortened if technique is traumatic and tissue thromboplastin is introduced to the sample and if collection tube is not filled properly.

  5. Influence of prescribed medications: antibiotic drugs, aspirin, cimetidine, isoniazid, phenothiazines, cephalosporins, cholestyramines, phenylbutazone, metronidazole, oral hypoglycemic drugs, phenytoin.

  6. Prolonged storage of plasma at 4 °C activates factor VII and shortens PT.