Prothrombin Time

Prothrombin Time (PT)

Normal Values
11.0–13.0 sec (can be variable according to laboratory methodology)

Therapeutic Levels
Therapeutic levels are at a prothrombin time ratio of 2.0–2.5.

Background
Prothrombin is a protein, produced by the liver that acts in the clotting of blood. Prothrombin production depends on adequate vitamin K intake and absorption. During the clotting process, prothrombin is converting to thrombin. The prothrombin content of the blood will be reducing in patients with liver disease.
Explanation of Test
Prothrombin time 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 through analysis of the clotting ability of five plasma coagulation factors (prothrombin, fibrinogen, factor V, factor VII, and factor X). This is know as the “prothrombin time” and is commonly ordered during management of oral anticoagulant (warfarin; Coumadin) therapy.

Procedure
1. A 5-ml venous blood sample is drawn (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. Blue top vacuum tubes will keep prothrombin levels stable at room temperature if left capped.

Oral Anticoagulant Therapy
Oral anticoagulant drugs (eg, warfarin [Coumadin] and dicumarol) are commonly prescribed to treat blood clots. These are indirect anticoagulants (as compared with heparin, which is a direct anticoagulant). However, if necessary, heparin is the initial choice for initiating treatment because it acts rapidly and partially lyses the clot.
1. These drugs act through the liver to delay coagulation by interfering with the action of vitamin K–related factors (II, VII, IX, and X) that promote clotting.
2. Oral anticoagulants delay vitamin K formation and cause prothrombin time to increase owing to decreased factors II, VII, IX, and X.
3. The usual procedure is to run a PT test every day. Once the PT is determined, the anticoagulant dose is adjusted until the therapeutic range is reached. Then, weekly to monthly, PT testing continues for the duration of therapy.
4. Warfarin (Coumadin) takes 48 to 72 hours to cause a measurable change in the PT.

Drug Therapy and PT Protocols
1. Patients with cardiac problems are usually maintained at a PT level 2 to 2.5 times normal (baseline) values.
2. Using the INR values allows more sensitive control.
3. For treatment of blood clots, the PT is maintained within 2 to 2.5 times normal. 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.

Clinical Implications
1. Conditions showing increased PT levels include
• Prothrombin deficiency (factor II) also factors V, VII, X
• Vitamin K deficiency
• Hemorrhagic disease of the newborn
• Liver disease (eg, alcoholic hepatitis)
• Current anticoagulant therapy outside the therapeutic range
• Biliary obstruction
• Salicylate intoxication
• Hypervitaminosis A
• Disseminated intravascular coagulation (DIC)
• Zollinger–Ellison syndrome
• Hypofibrinogenemia (factor I deficiency)
• Systemic lupus erythematosus (SLE)
2. Conditions that decrease the PT include
• Ovarian hyperfunction
• Regional enteritis/ileitis
3. Conditions that do not affect the PT include
• Polycythemia Vera
• Tannin disease
• Christmas disease (factor IX deficiency)
• Hemophilia A (factor VIII)
• Von Willebrand’s deficiency
• Platelet disorders—idiopathic thrombocytopenic purpura (ITP)

Interfering Factors
1. Diet: ingestion of excessive green, leafy vegetables (increases the body’s absorption of vitamin K, which promotes blood clotting).
2. Alcoholism and excessive alcohol ingestion raise PT levels.
3. Diarrhea and vomiting decrease PT because of dehydration.
4. Quality of venipuncture: PT can be shortened if technique is relatively traumatic.
5. Influence of prescribed medications (eg, isoniazid [INH], phenothiazides, cephalosporins, cholestyramines, phenylbutazone, metronidazole, oral hypoglycemics, and phenytoin).

Patient Preparation
1. Explain the purpose, procedure, and need for frequent testing. Emphasize the need for regular monitoring through frequent blood testing if on long-term therapy. 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 quickly 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 the anticoagulants and the PT values. Aspirin, acetaminophen, and laxative products should be avoided unless specifically ordered by the physician.
3. Instruct the patient never to start or to discontinue any drug without the doctor’s permission. This will affect PT values and may interfere with the healing process.
4. Counsel about diet. Excessive amounts of green, leafy vegetables (eg, spinach, broccoli) will increase vitamin K levels and could interfere with anticoagulant metabolism. Caution against using razor blades for shaving, trimming, and such. Electric shavers should be used.
5. These guidelines also apply to aftercare.




Patient After care
1. Interpret test outcomes and monitor appropriately.
2. Avoid intramuscular injections during anticoagulant therapy because hematomas may form at the injection site. As the PT increases to upper limits (> 30 minutes), carefully assess for bleeding from different areas. It may be necessary to perform neurologic assessments (if cranial bleeding is suspected), lung assessments and auscultation, GI and GU assessments, or other assessments as appropriate. Instruct the patient to observe for bleeding from gums, in the urine, or other unusual bleeding. Advise that care should be exercised in all activities lest accidental injury occur.
3. Patients who are being monitored by PT for long-term anticoagulant therapy should not take any other drugs unless specifically prescribed.
4. When unexpected adjustments in anticoagulant dosages 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. Changes in exercise intensity should be done gradually or even avoided. Active sports and contact sports should be avoided because of potential for injury.


Critical Value

1. If P/C ratio is greater than 2.5 or longer than 30.0 seconds, notify physician.
2. If PT is excessively prolonged (> 40 sec), vitamin K may be ordered.
3. Baseline PT levels should be drawn before anticoagulant administration.