Prothrombin Time

Purpose

Screening test to indentify acquired or inherited deficiencies in the activities of factors VII, X, V, prothrombin and fibrinogen.
Monitor oral anticoagulant therapy with warfarin, which decreases the activity of VII, IX, X and prothrombin.

Method

Collect blood from the patient into a citrated tube
Take the collected blood to the laboratory
Centrifuge to isolate the plasma
Take small sample of the plasma ~0.1 ml.
Add calcium
Add Thromboplastins - which are preparations of tissue factor and phospholipids from rabbit brain or more recently provided by genetic engineering.
The time that elapses until clot forms is the activated Prothrombin Time (PT).

Interpretation

PT shorter than the reference range is not associated with any clinical condition and is usually related to improper procedure.
Prolongation of thePT occurs when there is:

- Increased hematocrit, leading to a false positive in PT prolongation.
- Liver disease
- Vitamin K deficiency
- Disseminated intravascular coagulation
- Nephrotic Syndrome
- Treatment with certain antibiotics, chemotherapeutics, or antithrombotic drugs.

Note: The PT will increase shortly after a bolus dose of heparin.
The monitor of anticoagulant therapy requires standardization of the PT between different laboratories in order to ensure that the degree of anticoagulation measured in one laboratory can be compared to that in another laboratory. Therefore, the INR was created. The INR takes into account variations in reagents and techniques between different labs and provides one value that can be compared between labs.

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