Prothrombin Time (PT)
|Why is this test done?||This test is used to identify certain coagulation factor deficiencies. It is part of warfarin (CoumadinTM) monitoring - in this case it is expressed as a normalized ratio (INR).|
|How to prepare:||
|Associated Tests:||Other tests are often done at the same time, for example coagulation factors, partial thromboplastin time , and fibrinogen assay.|
Usually the blood is fluid and circulates in the vessels. If a vessel is damaged, it is important that the blood does not leak through the damaged area. To limit blood loss, a series of complex processes are activated. This is called coagulation.
Coagulation includes a cascade of enzymatic reactions involving coagulation factors, platelets, damaged wall cells, other blood protein, etc. Coagulation factors are assigned a number ranging from I to XIII. Each factor exists under an inactive and an activated form. Coagulation involves two pathways, one intrinsic the other extrinsic, which end in a final common pathway.
Coagulation is a normal and essential process. Various situations can lead to abnormal coagulation. Excessive coagulation increases the risk of thrombosis, while reduced coagulation can lead to haemorrhage.
Several tests can be performed to assess coagulation. Prothrombin time is used to measure the time it takes to form a clot under certain conditions. This test is used to evaluate the extrinsic pathway.
INR refers to International Normalized Ratio. Prothrombin time from a person is compared to normal results using a calculation.
What does an abnormal test result mean?
If the result is too high
A deficiency in vitamin K or certain coagulation factors can prolong the prothrombin time. Liver problems (hepatitis, cirrhosis, etc.) as well as salicylate (AspirinTM) overdose can also prolong the prothrombin time. This time as combined with INR results are also used to adjust the dosage of certain drugs, such as CoumadinTM or nicoumalone (SintromTM). These drugs affect the synthesis of certain vitamin K-dependent coagulation factors from the extrinsic pathway.
Factors that can affect the result of the test
Many factors can affect INR results, either downward or upward, in people taking anticoagulants. Here are a few examples:
- changes in the diet (more or less green vegetables and fruits);
- missed drug dose;
- interaction with OTC or prescription medication;
- diarrhea, etc.
What you need to know before the test
Before going for blood tests, a procedure or other exam, it is best to always bring a list of all the drugs you take (prescription, OTC and natural health products). Unless told otherwise, you should take your medication as usual on the day of the test. When in doubt, ask your pharmacist for more information.
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The patient information leaflets are provided by Vigilance Santé Inc. This content is for information purposes only and does not in any manner whatsoever replace the opinion or advice of your health care professional. Always consult a health care professional before making a decision about your medication or treatment.