Prothrombin Time Test

Prothrombin Time Test (PTT) | International Normalised Ratio (INR) | Procedure | Result

What is meant by Prothrombin Time Test (PT) / International Normalised Ratio (INR)?

The prothrombin time (PT) test or pro time test measures how long a blood sample takes to become a clot. International normalized ratio (INR), a type of calculation that is used to correlate with PT. 

Clotting is important to prevent excessive bleeding. Some proteins (called clotting factors) are necessary for proper blood clotting. If these clotting factors are insufficient, or if any of them are not functioning properly, then the blood clotting may take longer than normal.

What is prothrombin?

Prothrombin is a blood clotting factor synthesized by the liver. A cascade of reactions begins in the blood when the blood vessel is damaged. Blood clots form at the site of injury, and a strong blockage is formed at the site of damage, stopping the bleeding within a few seconds. Time is an important factor in the clotting process to prevent excess blood from the body. Inadequate functioning of any clotting factors can interfere with the clotting process and put a person at risk for excessive blood loss.

A blood clot usually occurs within a few seconds. If the person has a pre-existing condition then this may be more. If a person is taking medications that interfere with clotting, such as warfarin, it can also increase clotting time.

What is the use of this Test?

The PT / INR test is most commonly used:

  • To see how well warfarin works. Warfarin is a blood-thinning drug used for the treatment and prevention of dangerous blood clots. (Coumadin is a common generic brand name of warfarin.)
  • Find out the cause of abnormal blood clots.
  • Find out the cause of abnormal bleeding.
  • Check blood clotting work before surgery.
  • Check for liver problems.

The PT / INR test is often performed in combination with the partial thromboplastin time (PTT) test. The PTT test also checks for clotting problems.

Why is it essential to take this Test?

If you are taking warfarin regularly then you may need this test. The test helps you ensure that you are taking the correct dosage.

If you are not taking warfarin, you may need this test if you have symptoms of a bleeding disorder or bleeding disorder.

Symptoms of a bleeding disorder include:

  • Unexplained heavy bleeding
  • Rises easily
  • Unusually heavy notched
  • Abnormally heavy menstruation in women

Symptoms of clotting disorder include:

  • Leg pain or tenderness
  • Swelling of the feet
  • Redness or red lines on feet
  • labored breathing
  • cough
  • Pain in chest
  • fast heartbeat

In addition, if you are scheduled for surgery, you may require a PT / INR analysis. This helps to ensure that your blood is forming a clot normally so that you do not lose too much blood during the procedure.

How to Prepare for the PT test?

The PT test does not require any special training. Inform your doctor if you are taking any medicines or herbal supplements as they can be discontinued before taking this test.

What happens during the PT / INR exam?

The test can be performed on a blood sample from a vein or with a fingertip.

For blood test from the vein:

The phlebotomist will take a blood sample from a vein in your arm using a small needle. After pricking a very small amount of blood will be collected in a test tube or vial. You may feel a slight tingle as soon as the needle comes in or exits. It is a fast and easy procedure and usually takes less than five minutes.

For a fingertip blood test:

A fingertip test can be done at a health care provider’s office or at your home. If you are taking warfarin, your healthcare provider may suggest that you get your blood regularly from the home PT / INR test kit. 

During this test procedure, the healthcare provider will:

  • Pierce the tip of your finger with a small needle.
  • Collect a drop of blood and place it on a test strip or other special device.
  • Place the instrument or test strip in the device that is calculating the results. Household appliances are small and lightweight.

If you are using a home test kit, you will need to discuss your results with your healthcare provider. Your doctor will tell you how he wants to get the results.

What do the results mean?

If you are testing because you are taking warfarin, then your results are likely to be reported as INR levels. INR levels are often used because they make it easier to compare results from different laboratories and different testing methods. If you are not taking warfarin, your results can be in the form of INR level or number of seconds for blood sample clotting (prothrombin time).

If you are taking warfarin:

An INR that is too low may mean that you are at risk of dangerous blood clots.

An INR level that is too high may mean that you are at risk of dangerous bleeding.

Your health care provider will change your warfarin dose to reduce these risks.

If you are not taking warfarin and your INR or prothrombin time results are abnormal, it may mean one of the following:

  • Blood clotting disorder, a condition in which the blood clot does not settle in the body, causing excessive bleeding.
  • A blood clotting disorder, a condition in which the body makes excessive amounts of clots in the arteries or veins.
  • Liver disease

(Disclaimer: Information provided in this piece of article is purely for educational purposes only. All results must be clinically correlated with the patient’s data to make an accurate diagnosis.) 

References :

  1. Medplus, US National Library of Medicine ; Prothrombin Time (PTT)
  2. Lab Test Online, (internet) ; Prothrombin Time and International Normalised Ratio (PT/INR)
  3. ( internet) ; Prothrombin Time Test


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