Deep vein thrombosis (DVT) sometimes occurs for no apparent reason. However, the risk of developing DVT is increased in certain circumstances.
Inactivity
When you are inactive, your blood tends to collect in the lower parts of your body, often in your lower legs. This is usually nothing to worry about because when you start to move, your blood flow increases and moves evenly around your body.
However, if you are immobile (unable to move) for a long period of time, such as after an operation, due to an illness or injury or during a long journey, your blood flow can slow down considerably. A slow blood flow increases the chances of a blood clot forming.
In hospital
Because DVT is more likely to happen when you are unwell or inactive, or less active than you usually are, people in hospital are at a higher risk of getting a blood clot.
As a patient, your risk of developing DVT depends on the type of treatment you are having. You may be at higher risk of DVT if any of the following apply:
- You are having an operation that takes longer than 90 minutes, or 60 minutes if the operation is on your leg, hip or abdomen.
- You are having an operation for an inflammatory or abdominal condition such as appendicitis.
- You are confined to a bed, unable to walk, or spending a large part of the day in a bed or chair for at least three days.
If you are much less active than usual because of an operation or serious injury, and have other DVT risk factors such as a family history, you may also be at a higher risk of DVT.
When you are admitted to hospital you will be assessed for your risk of developing a blood clot and, if necessary, given preventative treatment.
Blood vessel damage
If the wall of a blood vessel is damaged, it may become narrowed or blocked, which can result in the formation of a blood clot.
Blood vessels can be damaged by injuries such as broken bones or severe muscle damage. Sometimes, blood vessel damage that occurs during surgery can cause a blood clot, particularly in operations on the lower half of your body.
Conditions such as vasculitis (inflammation of the vein wall), varicose veins and some forms of medication, such as chemotherapy, can also damage blood vessels.
Medical and genetic conditions
Your risk of DVT is increased if you have a condition that causes your blood to clot more easily than normal. These conditions include:
- cancer (treatments such as chemotherapy and radiotherapy can increase this risk further)
- heart and lung disease
- infectious diseases such as hepatitis
- inflammatory conditions such as rheumatoid arthritis
- thrombophilia (a genetic condition that makes your blood more likely to clot) and
- Hughes syndrome (when your blood becomes abnormally "sticky")
Pregnancy
Pregnancy makes your blood clot more easily. This is your body's way of preventing too much blood loss during childbirth. Around one in 1,000 pregnant women develops DVT at some point during their pregnancy (see DVT - Helen's story for an example of this).
Contraceptive pill and hormone replacement therapy (HRT)
The combined contraceptive pill and hormone replacement therapy (HRT) both contain the female hormone oestrogen. Oestrogen causes the blood to clot slightly more easily, so your risk of getting DVT is slightly increased. There is no increased risk from the progestogen-only contraceptive pill.
Other causes
Your risk of developing DVT is also increased if you or a close relative have previously had DVT, and if you are:
- overweight or obese
- a smoker
- dehydrated
- over 60 (particularly if you have a condition that restricts your mobility)
