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PATIENT INFORMATION
CLA: Prothrombin Time
BLOOD CLOTTING
When a blood vessel is damaged, a dizzying number of chemical reactions occur in the blood at that site which, under normal circumstances, produces a blood clot. Sometimes, the clotting process starts even without damage to a blood vessel. And in some clinical circumstances, your doctor prescribes a “blood-thinning” medication to prevent blood clotting in your blood vessels. The clinical laboratory performs tests to evaluate the functioning of many of the chemicals involved in blood clotting. One of these chemicals that is critical to the blood clotting process and, in particular, to that part of the process which is influenced by your “blood-thinning” medication (for example, Coumadin), is called prothrombin. The test that measures its function (and several others as well) is called the prothrombin time, “pro-time” or “PT” test. Your medication causes your liver to produce less prothrombin; with less prothrombin in your blood, your blood takes longer to clot and this amount of time for clotting is measured against the clotting time of normal blood.
TEST RESULTS
The PRL reference range (range within which most normal people’s PT test results will be) is 12.4 – 14.6 seconds. If you are taking a “blood-thinning” medication, your physician will have a PT test time (in seconds) in mind that he/she wants your PT result to be near.
The amount of time it takes for blood to clot in the PT test is very dependent upon the specific chemicals added to your blood in the test, so the time taken to clot can actually vary slightly between laboratories. With this variability in mind, modern clinical laboratories do some additional standardization of PT test results so that their test results will be comparable to those of other clinical laboratories using the same standardization method. The result is what is called an INR (International Normalized Ratio) value. INR reference range values are normally around 1.
YOUR TEST RESULTS
Your physician will have determined a range of INR values within which he/she wants your INR value to be. Each PT test PRL performs, then, will give both a number of seconds that it took for your blood to clot in the test, and an INR value. You should have instructions from your physician as to how to use this information in maintaining the correct dosage of your “blood-thinning” medication.
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