An abnormal tendency for blood to clot inside blood vessels

  • Gender, genetics, and lifestyle as risk factors depend on the cause
  • Age is not a significant factor

Blood clotting normally occurs only when a blood vessel is damaged. The clotting process relies on blood cells called platelets and on proteins in the blood known as clotting factors. Other substances in the blood prevent spontaneous blood clotting or help to dissolve clots. Excessive clotting (hypercoagulability) may result if the balance of clotting and anticlotting mechanisms is upset or if blood flow is slowed down.

Increased blood clotting may be life-threatening. A clot that forms in a vein (see Deep vein thrombosis) may travel to the lungs and block an artery (see Pulmonary embolism). Clots may also block an artery in the heart, causing a heart attack (see Myocardial infarction), or in the brain, causing a stroke.

What are the causes?

Hypercoagulability may be caused by inherited disorders in which a particular anticlotting substance (proteins C or S, or antithrombin) is deficient, or an abnormal form of the blood clotting Factor V is produced, as in the inherited disorder known as Factor V Leiden.

Another cause of hypercoagulability is increased “stickiness” of platelets due to smoking or diabetes mellitus. In women, the disorder may also be a result of changes in the levels of female sex hormones, as may occur in pregnancy, while taking the combined contraceptive pill, and with hormone replacement therapy (HRT).

Raised levels of red blood cells (see Polycythaemia) can increase the thickness of the blood, leading to hypercoagulability. Immobility, such as may occur during a long flight or when confined to bed during an illness or after surgery, can also increase the likelihood of hypercoagulability.

What might be done?

If you have a family history of hypercoagulability or develop a blood clot, your doctor may arrange for you to have blood clotting tests. If an inherited disorder such as Factor V Leiden is diagnosed, your doctor may prescribe drugs that prevent blood clotting and may also advise genetic counselling. Whatever the cause, you should avoid smoking or taking drugs that contain oestrogen, such as combined oral contraceptives and HRT.

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

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