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Blood Clotting Tests

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, etc. Blood cells, which can be seen under a microscope, make up about 40% of the blood's volume.

Blood cells are divided into three main types.

  • Red cells (erythrocytes). Millions are released into the bloodstream from the bone marrow each day. Red cells contain a chemical called hemoglobin. Hemoglobin is attracted to oxygen and the two substances can bind together. This allows oxygen to be transported by red blood cells to all parts of the body.
  • White cells (leukocytes). There are different types of white cells such as neutrophils (polymorphs), lymphocytes, eosinophils, monocytes, basophils. They are a part of the immune system and are mainly involved in combating infection.
  • Platelets. These are tiny and help the blood to clot if we cut ourselves

How does blood clot?

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 and proteins in the plasma, called clotting factors (Factor I to XIII). There are 13 known clotting factors. 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.

In the normal state 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.

Bleeding disorders

  • Too few platelets (thrombocytopenia) - due to various causes.
  • Genetic conditions where you do not make one or more clotting factors. The most well known is haemophilia A which occurs in people who do not make factor VIII.
  • Lack of vitamin K, which can cause bleeding problems, as you need this vitamin to make certain clotting factors.
  • Liver disorders - these sometimes cause bleeding problems, as your liver makes most of the clotting factors.

Clotting disorders

  • A blood clot that forms within an artery supplying blood to the heart or brain is the common cause of heart attack and stroke.
  • Sluggish blood flow can make the blood clot more readily than usual as in deep vein thrombosis (DVT) which is a blood clot that sometimes forms in a leg vein.
  • Certain genetic conditions can make the blood clot more easily.
  • Certain medicines can affect the blood clotting mechanism which may result in the blood clotting more readily.
  • Liver disorders can sometimes cause clotting problems.

Blood clotting tests

  • If you have a suspected bleeding disorder.
  • If you have certain liver diseases that can affect the making of blood clotting factors.
  • Before surgery, in certain circumstances, to assess your risk of bleeding problems during an operation.
  • If you develop a blood clot within a blood vessel for no apparent reason.
  • If you take anticoagulant medication such as warfarin (to check that you are taking the correct dose).

Blood count

A full blood count is a routine blood test that can count the number of red cells, white cells and platelets per ml of blood. It will detect a low level of platelets.

Bleeding time

In this test, a tiny cut is made in your earlobe or forearm and the time taken for the bleeding to stop is measured. It is normally 3-8 minutes.

General blood clotting tests

The 'prothrombin time' (PT) and the 'activated partial thromboplastin time' (APTT) are commonly done. These tests measure the time it takes for a blood clot to form after certain activating chemicals are added to the blood sample. If the time taken is longer than for a normal blood sample then one or more clotting factors are absent or low.

Monitoring anticoagulants

If you take certain anticoagulants (medicines which reduce the chance of a blood clot forming) then you need careful monitoring. Too much of the medication may cause bleeding problems. Too little medication may increase the chance that a clot may form. A measurement called the INR can monitor how much medicine (commonly warfarin) to take. Your INR is calculated by the laboratory using the PT mentioned above. By checking your blood at regular intervals they can advise on how to adjust your dose of medication to reach this target.

Specific blood clotting factors

The amount of various clotting factors (and anti-clotting factors) in the blood can be measured by various techniques. For example, the amount of factor VIII can be measured in a blood sample. (The level is very low or absent in people with hemophilia 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.


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