A deep vein thrombosis (DVT) is a blood clot in a vein. Blood clots in veins most often occur in the legs but can occur elsewhere in the body, including the arms. This leaflet is about blood clots in leg veins.
The most common cause of a blood clot developing in a vein is immobility. A complication can occur in some cases where part of the blood clot breaks off and travels to the lung (pulmonary embolus). This is usually prevented if you are given anticoagulation treatment.
A DVT is a blood clot that forms in a deep leg vein. Veins are blood vessels that take blood towards the heart.
Deep leg veins are the larger veins that go through the muscles of the calf and thighs. They are not the veins that you can see just below the skin, neither are they the same as varicose veins. When you have a DVT, the blood flow in the vein is partially or completely blocked by the blood clot.
A calf vein is the common site for a DVT. A thigh vein is less commonly affected. Rarely, other deep veins in the body can be blocked by blood clots.
A DVT is part of a group of problems together known as venous thromboembolism (VTE).
Venous means related to veins. A thrombosis is a blockage of a blood vessel by a thrombus (a blood clot). An embolism is where the thrombus dislodges from where it formed and travels in the blood until it becomes stuck in a narrower blood vessel, elsewhere in the body. The thrombus is then called an embolus.
A pulmonary embolism (PE) is where a thrombus has broken off from a DVT (usually in the leg) and become stuck in one of the blood vessels in the lung. Pulmonary emboli is also part of venous thromboembolism. (See separate leaflet called Pulmonary Embolism for more information.)
Blood normally flows quickly through veins, and does not usually clot. Blood flow in leg veins is helped along by leg movements, because muscle action squeezes the veins. Sometimes a DVT occurs for no apparent reason. However, the following increase your risk of having a DVT:
It is estimated that about 1 in 1,000 people have a DVT each year in the UK.
A DVT most commonly develops in a deep vein below the knee in the calf. Typical DVT symptoms include:
Sometimes there are no symptoms and a DVT is only diagnosed if a complication occurs, such as a pulmonary embolus (see below).
Sometimes it is difficult for a doctor to be sure of the diagnosis from just your symptoms, as there are other causes of a painful and swollen calf. Examples of conditions that can cause similar symptoms are muscle sprains or skin infections (cellulitis). Your doctor might calculate something called a Well's score to work out the likelihood of you having a DVT. It involves looking at your symptoms and risk factors for a DVT.
If you have a suspected DVT, you will normally be advised to have tests done urgently to confirm or rule out the diagnosis. Two commonly used tests are:
Sometimes these tests are not 100% conclusive and more detailed tests are necessary. Contrast venography is another test that can be done. In this test a dye is injected into the leg veins. X-ray tests can then detect the dye which is shown not to be flowing if a vein is blocked by a clot.
Sometimes, especially if there is a delay in getting a scan, you may be given daily injections of heparin. This is, in effect, treating you as if you do have a DVT, even though it has not been proven. This is safer than doing nothing whilst waiting for a scan.
It can be. When a blood clot forms in a leg vein it usually remains stuck to the vein wall. The symptoms tend to settle gradually. However, there are two main possible complications:
In a small number of people who have a DVT, a part of the blood clot breaks off. This travels in the bloodstream and is called an embolus. An embolus will travel in the bloodstream until it becomes stuck. An embolus that comes from a clot in a leg vein will be carried up the larger leg and body veins to the heart, through the large heart chambers, but will get stuck in a blood vessel going to a lung. This is called a pulmonary embolus.
DVTs and PEs are known collectively as venous thromboembolism (VTE).
A small PE may not cause any symptoms. A medium-sized PE can cause breathing problems and chest pain. A large PE can cause collapse and sudden death. It is estimated that about 1 in 10 people with an untreated DVT develop a PE large enough to cause symptoms or death.
Without treatment, up to 6 in 10 people who have a DVT develop long-term symptoms in the calf. This is called post-thrombotic syndrome. Symptoms occur because the increased flow and pressure of the diverted blood into other veins can affect the tissues of the calf. Symptoms can range from mild to severe and include: calf pain, discomfort, swelling, and rashes. An ulcer on the skin of the calf may develop in severe cases.
Post-thrombotic syndrome is more likely to occur if the DVT occurs in a thigh vein, or extends up into a thigh vein from a calf vein. It is also more common in people who are overweight, and in those who have had more than one DVT in the same leg.
The aims of treatment are:
Anticoagulation is often called thinning the blood. However, it does not actually thin the blood. It alters certain chemicals in the blood to stop clots forming so easily. It doesn't dissolve the clot either (as some people incorrectly think). Anticoagulation prevents a DVT from getting larger, and prevents any new clots from forming. The body's own healing mechanisms can then get to work to break up the clot.
Warfarin is the usual anticoagulant. However, it takes a few days for warfarin tablets to work fully. Therefore, heparin injections (often given just under the skin) are used alongside warfarin in the first few days (usually five days) for immediate effect. There are different brands of heparin injection, the common ones you might see used are Clexane® and Fragmin®. A serious embolus is rare if you start anticoagulation treatment early after a DVT.
The aim is to get the dose of warfarin just right so the blood will not clot easily, but not too much, which may cause bleeding problems. You will need regular blood tests (called INRs) whilst you take warfarin. The INR (which stands for International Normalised Ratio) is a blood test that measures your blood clotting ability. You need the tests quite often at first, but then less frequently once the correct dose is found. An INR of 2.5 is the aim if you have warfarin for a DVT, although anywhere in the range 2-3 is OK. If you have had recurrent DVTs, or have had a PE whilst on warfarin, you might need a higher INR (even 'thinner' blood).
A new drug called fondaparinux sodium (Arixtra®) can be given by injection in some circumstances, either to prevent or to treat a DVT or PE.
If you are pregnant, regular heparin injections rather than warfarin tablets may be used. This is because warfarin can potentially cause harm (birth defects) to the unborn child.
The length of time you will be advised to take anticoagulation depends on various factors. If you have a DVT during pregnancy or after an operation, then after the birth, or when you are fit again, the increased risk is much reduced. So, anticoagulation may be only for a few months. On the other hand, some people continue to have an increased risk of having a DVT. In this case, the anticoagulation may be long-term.
As a guide, for a DVT that happens below the knee, you will need at least six weeks' warfarin. Usually 3-6 months of warfarin treatment is given in this situation. The length of time of anticoagulation varies from person to person. Your doctor or anticoagulant clinic will advise you how long your treatment will be for.
Note: you should not travel on any long journeys or travel by plane until at least two weeks after starting anticoagulant treatment. Travel within two weeks of a DVT is not recommended without seeking advice from a specialist - not least, because you will need regular blood tests soon after starting warfarin.
Most people who develop a DVT are advised to wear compression stockings. This treatment has been shown to reduce the risk of a recurrent DVT, and can also reduce the risk of developing post-thrombotic syndrome. You should wear the stockings each day, for at least two years. If you do develop post-thrombotic syndrome, you may be advised to wear the stockings for more than two years.
The best type of stockings (or elastic compression hosiery as they are known), are grade 3 strength. This means that they are able to squeeze the legs, with a certain degree of force. This means that they will feel pretty tight. This is normal. It will not be as comfortable as the socks, tights or stockings you are used to wearing, and they can take a bit of getting used to. Some people really find that they cannot tolerate grade 3 stockings, and so grade 2 stockings (which create less of a squeezing force) can be used instead. It is not as good as wearing the grade 3 stockings, but better than nothing.
Note: a compression stocking used following a DVT should be fitted professionally after an assessment and accurate measurement. Do not just buy over-the-counter support stockings or flight socks that may be the wrong class or size and which may potentially cause more damage. Your stockings will also need changing every 3-6 months.
If you are advised to wear a compression stocking, you should put it on each day whilst lying in bed before getting up. Wear it all day until you go to bed, or until you rest in the evening with the leg raised. Take the stocking off before going to bed. The slight pressure from the stocking helps to prevent fluid seeping into the calf tissues from the outer veins which carry the extra diverted blood following a DVT. The stocking also reduces, and may prevent, calf swelling. This in turn reduces discomfort and the risk of skin ulcers forming.
Sometimes other treatments may be considered - for example:
A DVT is often just a one-off event after a major operation.
However, some people who develop a DVT have an ongoing risk of a further DVT - for example, if you have a blood clotting problem, or continued immobility. As mentioned above, you may be advised to take anticoagulation (usually with warfarin) long-term. Your doctor will advise you about this.
Other things that may help to prevent a first or recurrent DVT include the following: