Blood Clotting Tests

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




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.


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.

Bleeding disorders


  • 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

Sometimes a blood clot forms within a blood vessel that has not been injured or cut - for example:

  • A blood clot that forms within an artery supplying blood to the heart or brain is the common cause of heart attack and stroke. The platelets become sticky and clump next to patches of atheroma (fatty material) in blood vessels and activate the clotting mechanism.
  • Sluggish blood flow can make the blood clot more readily than usual. This is a factor in deep vein thrombosis (DVT) which is a blood clot that sometimes forms in a leg vein.

You may be advised to have tests of blood clotting:

  • If you have a suspected bleeding disorder. For example, if you bleed a lot after cuts, or if you bruise easily.
  • 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).

There are a number of different tests. The ones chosen depend on the circumstances and the suspected problem. They include the following:

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

A blood sample is taken into a bottle that contains a chemical which prevents the blood from clotting. It is then analysed in the laboratory. There are a number of tests that may be done. For example, 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. There are other similar tests where different chemicals are added to the blood sample. The aim is to identify which clotting factor or factors are low or absent.

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. Your doctor or nurse will set a 'target' INR for you, depending on the reason why you are taking the medication. 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. 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
Document ID:
4749 (v39)
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