Clotting, or coagulation, of blood is a protective mechanism to stem hemorrhage. Many different proteins are involved in the cascade of coagulation. Abnormality occurring at any stage of the process can lead to prolonged bleeding. Without treatment, bleeding disorders can sometimes cause the death of affected animals.

One of the tests used to help determine a possible cause of a bleeding disorder is the prothrombin time (PT). Checking PT is indicated in any pet with a suspected coagulation problem. This test is also performed before procedures in which it is important to establish that blood clotting is adequate, e.g. liver biopsy.

There are no contraindications to performing this test. In pets with bleeding abnormalities, it is possible for venipuncture to cause excessive bleeding, so special care must be taken by a) drawing the blood from a small vein and b) applying prolonged pressure at the injection site.

What Does a Prothrombin Time Reveal?

Prothrombin time is a measure of the time it takes for blood plasma to clot. It is used to evaluate the proper functioning certain coagulation proteins. This test detects deficiencies in the extrinsic and common pathways. Of the 12 clotting proteins involved in forming a blood clot, PT evaluates Factors X, VII, V, II and I. Several of these clotting factors rely on vitamin K for proper function. Normal PT in cats is less than 22 seconds, but this can vary from laboratory to laboratory. Values greater than 22 seconds indicate a potential bleeding disorder. The most common cause of a prolonged prothrombin time is poisoning with an anticoagulant rodenticide.

How Is a Prothrombin Time Done?

In order to check PT, your veterinarian must first draw a blood sample, which is then placed in a special glass tube. This sample is often submitted to a laboratory for analysis. Some veterinary clinics, and most veterinary emergency facilities, have the capability to perform this test in their hospital. Others rely on outside laboratories. In emergency situations, PT may be run at a local human hospital for rapid results. The test typically takes less than 10 minutes to run once the blood is available but, if submitted to an outside laboratory, the test results may not be available for 1 to 2 days.

Is a Prothrombin Time Painful?

The only pain involved is associated with the collection of the blood sample. The pain associated with this process varies from individual to individual, as it does in humans.

Is Sedation or Anesthesia Needed for a Prothrombin Time?

Neither sedation nor anesthesia is needed in most patients. However, some pets resent needle puncture and may benefit from sedation.