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Blood Clotting Tests

Blood clotting tests are used to diagnose and assess bleeding problems and to monitor people who take warfarin or other anticoagulant medicines.

Plasma, the liquid part of blood, makes up about 60% of the blood's volume. Plasma is mainly made from water but contains many different proteins and other chemicals such as hormones, antibodies, enzymes, glucose, fat particles, salts, etc.

Blood cells, which can be seen under a microscope, make up about 40% of the blood's volume. Blood cells are made in the bone marrow by blood 'stem' cells. Blood cells are divided into three main types.

Within seconds of cutting a blood vessel, the damaged tissue causes platelets to become 'sticky' and clump together around the cut. These 'activated' platelets and the damaged tissue release chemicals which react with other chemicals and proteins in the plasma, called clotting factors. There are 13 known clotting factors which are called by their Roman numbers - factor I to factor XIII. A complex cascade of chemical reactions involving these clotting factors quickly occurs next to a cut.

The final step of this cascade of chemical reactions is to convert factor I (also called fibrinogen - a soluble protein) into thin strands of a solid protein called fibrin. The strands of fibrin form a meshwork and trap blood cells and platelets which form into a solid clot.

If a blood clot forms within a healthy blood vessel it can cause serious problems. So, there are also chemicals in the blood that prevent clots from forming and chemicals that 'dissolve' clots. There is balance between forming clots and preventing clots. Normally, unless a blood vessel is damaged or cut, the 'balance' tips in favour of preventing clots forming within blood vessels.

Within seconds of cutting a blood vessel, the damaged tissue causes platelets to become 'sticky' and clump together around the cut. These 'activated' platelets and the damaged tissue release chemicals which react with other chemicals and proteins in the plasma, called clotting factors. There are 13 known clotting factors which are called by their Roman numbers - factor I to factor XIII. A complex cascade of chemical reactions involving these clotting factors quickly occurs next to a cut.

The final step of this cascade of chemical reactions is to convert factor I (also called fibrinogen - a soluble protein) into thin strands of a solid protein called fibrin. The strands of fibrin form a meshwork and trap blood cells and platelets which form into a solid clot.

If a blood clot forms within a healthy blood vessel it can cause serious problems. So, there are also chemicals in the blood that prevent clots from forming and chemicals that 'dissolve' clots. There is balance between forming clots and preventing clots. Normally, unless a blood vessel is damaged or cut, the 'balance' tips in favour of preventing clots forming within blood vessels.

Specific blood clotting factors

The amount of various clotting factors (and anti-clotting factors) in the blood can be measured by various techniques. One or more of these tests may be done if a general blood clotting test identifies a problem with clotting. For example, the amount of factor VIII can be measured in a blood sample. (The level is very low or absent in people with haemophilia A.)

Platelet aggregation test

This measures the rate at which, and the extent to which, platelets form clumps (aggregate) after a chemical is added which stimulates aggregation. It tests the function of the platelets.

Tests to check if your blood clots too easily

If you have an unexplained blood clot within a normal blood vessel, you may have tests to investigate possible causes. For example, a blood test to check for 'factor V Leiden'. This is an abnormal variation of factor V which tends to make the blood clot more readily than normal.

Other tests

Various conditions such as vitamin deficiencies, leukaemia, liver disorders, or infections may affect clotting. Therefore, in some cases other tests may be needed to find the cause of abnormal levels of platelets or clotting factors.

Original Author: Dr Tim Kenny Current Version: Peer Reviewer: Dr Hannah Gronow
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