The following info is from this site:
http://www.labtestsonline.org
I thought it was interesting, especially what it has to say about what things interfere with the INR results.
Prothrombin Time
Also known as: PT, Pro Time
Formally known as: Prothrombin Time
Related tests: Partial Thromboplastin Time (PTT), Activated Clotting Time (ACT), Activated Partial Thromboplastin Time (APTT), Prothrombin Consumption Time (PCT), Fibrinogen, Coagulation Factors
At A Glance
Why get tested?
To check how well blood-thinning medications (anti-coagulants) are working to prevent blood clots.
When to get tested?
If you are taking an anti-coagulant drug or think you may have a bleeding disorder
Sample required?
A blood sample drawn from a vein in the arm
Frequency of testing?
Regularly, as your doctor advises
The Test Sample
What is being tested?
The PT test measures how long it takes your blood to begin to form clots. Prothrombin is a plasma protein produced by the liver. Clotting is caused by a series of chemical reactions, including the conversion of prothrombin to thrombin. The test to measure this clotting factor is called Prothrombin time or Pro time or PT.
How is the sample collected for testing?
Blood is collected by needle from a vein in the arm.
The Test
How is it used?
The Prothrombin time (PT) test is most often used to check how well anti-coagulant or ?blood-thinning? drugs are working. Anti-coagulant drugs help prevent the formation of blood clots. This is particularly important in people who have had a heart attack or other problems with their veins. The drug?s effectiveness can be determined by how much it prolongs the prothrombin-to-thrombin conversion time.
When is it ordered?
If you are taking an anti-coagulant drug, your doctor will check your PT regularly to make sure that your prescription is working properly and that your PT is appropriately increased. There is no set frequency for doing the test. Your doctor will order them often enough to make sure that the drugs are in the appropriate dosages. Occasionally the PT test may be used on a patient who is not taking anti-coagulant drugs?to check for a bleeding disorder or to ensure clotting ability before surgery.
What does the test result mean?
The test result for PT depends on the method used; results will be measured in seconds.
Most laboratories report PT results that have been adjusted to the International Normalized Ratio (INR). Patients on anti-coagulant drugs should have an INR of 1.2 to 1.5 for basic ?blood-thinning? needs. For some patients who have a high risk of clot formation, the INR needs to be higher: about 1.5 to 2.0(***people with mechanical heart valves will need a much higher INR, possibly 2.5 to 3.5***, note from Nancy). Your doctor will use the INR to adjust your drug to get the PT into the range that is right for you. A prolonged, or increased, Prothrombin time means that your blood is taking too long to form a clot. If you are not taking anti-coagulant drugs and your PT is prolonged, additional testing may be necessary to determine the cause. Patients with liver disease often have a prolonged PT.
Is there anything else I should know?
Some substances you consume?such as alcohol and different drugs?can interfere with the PT test and give a misleading result. Antibiotics, aspirin, and cimetidine can increase PT. Barbiturates, oral contraceptives and hormone-replacement therapy (HRT), and vitamin K?either in a multivitamin or liquid nutrition supplement?can decrease PT. Make sure that your doctor knows all the drugs you are taking so that the PT results are interpreted correctly.
Common Questions
1. Can I do this test at home?
Yes. The Food and Drug Administration has recently approved a home test. Ask your doctor whether home testing is appropriate for you.
2. Should I have it done at the same time of day?
Yes. Not only should the test always be done at the same time of day, but anti-coagulant medication should be taken at the same time each day. This way, your doctor can get the most consistent and accurate assessment of your PT results and adjust your medication so that you are getting the correct dose.
3. My PT results vary sometimes, yet my doctor doesn?t change my prescription. Why?
The use of any of the drugs mentioned above can alter your results, as can the use of diuretics and antihistamines and the onset of illness or allergies. Certain foods, such as beef and pork liver, green tea, broccoli, chickpeas, kale, turnip greens, and soybean products contain large amounts of vitamin K and can alter PT results. The blood collection technique and the difficulty in obtaining the blood sample can also affect test results.
