Thousands of people will have long term health problems as a result of a blood clot in the vein. But many are completely unaware that they're at increased risk of venous thrombosis, missing out on treatment which could be life saving.

Clots can form in any vein deep within the body, but most often can be found in the deep veins of the leg. These deep vein thromboses (DVT) form in the calf or lower leg, behind the knee, in the thigh or in the veins passing through the pelvis.

If the DVT damages the delicate valves which help keep blood flowing upwards towards the heart, a condition called post-phlebitic syndrome may develop (occurring in one in five after a DVT). As a result, blood pools in the lower leg, increasing pressure in the vessels, causing swelling of the leg and ankle and a heavy sensation, especially after walking or standing. Skin ulcers may also develop.

When the clot forms, it blocks the vein, preventing blood from draining from the limb as it should. The result is that the leg becomes swollen and painful, may change colour (turning pale, blue or reddish-purple) or the skin appears tight or shiny.

More worrying is the risk that part of the clot will break away, forming what is known as an embolus which travels around the circulation, through the heart and into the lungs where it blocks a blood vessel. This is known as a pulmonary embolus (PE) and estimates suggest that as many as 50 per cent of those with a DVT will go on to develop a PE.

The symptoms of PE include chest pain and shortness of breath, which may be sudden and severe. Although some people develop a PE without noticing any symptoms, it can be extremely dangerous and cause damage to the lung tissues proving fatal in as many as one in ten unless treated.

Long term complications include chronic pulmonary hypertension, where the pressure in the blood vessels of the lung remains persistently high.

Symptoms of a DVT or PE may be minimal and can often go undiagnosed. When suspected, it can be difficult to reach a definite diagnosis because tests aren't straightforward and don't always give a clear result.

Once a DVT or PE has been diagnosed, treatment is started to thin the blood (known as anticoagulant therapy), reducing the risk that the clot will grow or spread.

Immediate treatment is given in the form of injections of a drug called heparin, and warfarin tablets, which take several days to build effect. When the cause of the clot formation is clear, such as after surgery, treatment is continued for three months, but when there's no obvious cause it may be continued for six months or even indefinitely.

The effects of warfarin can vary. Other illnesses and treatments, or even a change in diet, can interfere and it's quite easy to become over - or under - coagulated, leading to a risk of either a haemorrhage or further clots. Those taking warfarin must have regular blood tests to monitor their clotting levels, and the dose of warfarin adjusted accordingly.

Newer drugs are being developed which are more consistent in their effect and which one day may replace warfarin.

When someone is known to be at risk of venous thrombosis, clots can be prevented by taking a few simple steps and giving small doses of anti-coagulant drugs.

Heparin treatment will reduce the risk of DVT following cancer treatment or hip or knee surgery by 70 per cent. People having major surgery have a 30 per cent risk of DVT which is reduced by about 60 per cent with anti-coagulants.

If you're going into hospital for an operation or other treatment you should be assessed for your risk of developing a blood clot and in most cases will be offered preventative treatment with both compression stockings and anti-coagulant injections.