Learn about Coagulation Blood Tests

Learn about Coagulation Blood Tests
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How Blood Clots

Blood is made up of red blood cells, white blood cells, and platelets. Red blood cells transport oxygen to cells in your body and give blood its red color. White blood cells are part of the immune system and help fight infection. Platelets are tiny molecules that help blood clot.

As soon as a blood vessel is cut, the platelets begin to clump around the damaged tissue. The platelets and damaged tissue release chemicals, known as clotting factors, that start a complex chain of chemical reactions. The last step in the reaction is the formation of a protein called fibrin that binds blood cells and platelets into a clot.

Why Test Blood Coagulation?

There are several reasons why your doctor may order coagulation blood tests. If you experience excessive bleeding or bruising, you may have a bleeding disorder. This sort of problem can be inherited (like hemophilia) or acquired (such as vitamin K deficiency or liver disease). It is also possible that blood clots form inside your veins when you haven’t been cut, which is known as thrombosis. This can be caused by blocked arteries, slow blood flow, a liver disorder, or certain medications. Your doctor may also want to test your blood coagulation before a surgery to make sure your blood will clot at the necessary speed as required by the surgery.

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Coagulation Blood Tests

Complete Blood Count (CBC): This provides a look at all the types of cells in your blood. This will show if you have low platelets, which can be a cause of excessive bleeding.

Bleeding Time: This test is done by making a tiny cut, usually on the forearm or ear and measuring how long it takes for the cut to stop bleeding. The normal time is between three and eight minutes.

Prothrombin Time (PT) and Partial Thromboplastin Time (PTT): These tests require a small amount of blood to be drawn from a vein or sometimes from a prick on the finger. Activating chemicals are added to the blood sample in a lab to test how long it takes for a clot to form. If it takes longer than normal, it may indicate that certain clotting factors are missing in the blood. Normal PT times are 10-12 seconds, and normal PTT times are 30-45 seconds.

Platelet Aggregation: This test examines how well platelets are functioning by adding a chemical to the blood sample and measuring the rate and size of the clot formed.

Other tests can be run on the blood sample in addition to the ones listed above. If it seems that a clotting factor is missing from the blood, a test looking specifically at individual factors can be run.

If you are taking anticoagulants, the lab may look more closely at your blood and calculate a number called the International Normalized Ratio (INR). The INR is calculated using the PT results and can indicate to your doctor how to adjust the dosage of your medication.


1. https://surgery.about.com/od/beforesurgery/qt/PTPTTINRtests.htm

2. https://www.patient.co.uk/health/Blood-Test-Clotting-Tests.htm