A prothrombin test measures the time taken for the formation of blood clots, and it gives your doctor an idea that you might be suffering from a bleeding disorder, which affects certain clotting factors, which form a part of the clotting cascade. If you have been started on anti-coagulant medication, then the INR test will help your doctor determine if the correct dose is being administered.

The most common reason why your doctor will order these tests is to monitor how well your blood is clotting when you have been started on warfarin, an anticoagulant medication. You will be asked to undergo this test routinely, and if an abnormality is detected then it means that the dose of warfarin you are taking is either too les or too much. This test is also performed if you are diagnosed with liver disease or before you undergo surgery.

There are no special preparations required before you undergo this test, but if you are on anti-coagulant medication, then you should get tested before you take your daily dose. Sometimes you may be requested to omit some of the blood thinning agents you might be taking. Talk to your healthcare provider regarding this.

During this, a Medical Lab Technician will draw out a small volume of blood, using a needle and syringe which is inserted into a vein in your arm. This blood sample is then sent to the lab for analysis.

Since this test is a simple drawing of blood, you will not feel a lot of discomfort. You might experience a mild pain on inserting the needle, but you will not experience any uneasiness afterwards.

There is very little risk in undergoing a PT/INR test, because it is a simple blood drawing. But since this test is most often done on people with a bleeding disorder, there is an increased risk of excessive bleeding from the site of needle insertion. Very rarely you might continue to have a mild pain or some swelling and hematoma formation at the site where the needle was inserted, but these symptoms will resolve on their own within a couple of days.

The average time taken for blood to clot is between 10 to 14 seconds. Values less than that means your blood is clotting sooner than usual, and a value more than that means your blood is taking longer than usual to clot. Regarding the INR, if you are not on warfarin, then a reading of less than 1.1 is thought to be normal, and anything above that means your blood is taking longer to clot. For individuals on warfarin, the target INR range is 2-3.

As mentioned above, if you have a bleeding disorder, or if you have taken anticoagulant medication before undergoing this test, the results can be altered.

https://medlineplus.gov/ency/article/003652.htm https://www.webmd.com/a-to-z-guides/prothrombin-time-test#1 https://www.mayoclinic.org/tests-procedures/prothrombin-time/about/pac-20384661 https://labtestsonline.org/tests/prothrombin-time-and-international-normalized-ratio-ptinr