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Detailed Information on Deep Vein Thrombosis

This page is based on notes taken by Mr Braithwaite at a lecture given by Professor Frits Rosendaal at the Spring Meeting of the Venous Forum of the Royal Society of Medicine in March 2003


Doctors have known about venous thrombosis since the middle ages. It was first thought to be related to blood letting and infections. In 1759 Putos recognised the period after pregnancy, the puerperium, as a time of risk of a swollen leg. At that time 'milk leg', as it was called, was thought to be because excessive milk filled the leg. We now know that the hormonal changes of pregnancy cause a thickening of the blood so that it is more likely to clot. If the main veins of the leg are blocked then the drainage of the leg is worse and the leg swells. This would be like a blocked river causing flooding of surrounding fields.

In 1856 a Doctor called Virchow described the things that make it more likely that blood will clot in a vein or artery. These are a change in the flow of the blood, a change in what the blood is made from (the constituents), and a change in the wall of the blood vessel. These 3 factors are known as Virchow's triad.

For deep vein thrombosis, the flow of blood and constituents are the most important.

How common is Deep vein thrombosis?

If you were to study 1000 people from the time they are born to the time that they die then only one person would get a DVT.

What are the chances of having a pulmonary embolus?

A DVT in the leg can become dislodged and travel in the blood stream to the lung. This is called a pulmonary embolus (PE).

If 100 people had a DVT then 20 might get a PE. 

Of those 100 people only 1 would be likely to die because of the PE. In some people with severe illnesses perhaps 10 of the 100 people with a DVT would die because of a DVT..

So, in your lifetime, the chance of being killed by a pulmonary embolus is probably between 1 in 100,000 and 1 in 10,000. If you live to be 80 years old then your risk every year is 1 in 8 million. You are probably more likely to win the lottery.

Does Deep vein thrombosis cause problems other than a pulmonary embolus?

Nearly half of the people who have a DVT end up with a permanently swollen leg. Perhaps one quarter develop ulcers (sores) around their ankles. If this happens then it is advisable to see a vascular surgeon for advice. If you have had a DVT and wish to discuss your symptoms contact Mr Braithwaite by clicking

What things increase the risk of a DVT?

There are several things or risk factors:     Immobility, recent surgery, Age, Genetic factors, medicines and others.


If you sit down for a long time without moving, blood in your veins flows more slowly than when you walk around. If you sit with your knees and hips bent then the curves created by the blood vessels make the blood flow more slowly. Slow blood flow is one of the causes of a DVT.

How do I stop the blood flowing slowly?

When you use the muscles in your calf, they squeeze the veins and help to push the blood along. This is called the calf muscle pump. If you walk around then you will straighten the curves of the veins and pump the blood.

If your legs are raised up so that your feet are level with your tummy then the blood will run more easily.


The older we get, the more likely we are to get a DVT. When we are born our risks of having a DVT are extremly low, as we get older then we get closer to a 1 in 1000 chance of having a DVT. It is not understood why getting older should increase the risk of a DVT.


There are some inherited conditions that increase the risk of blood clots. To explain you need to understand how blood clots and is stopped from clotting.

How blood clots

As blood flows through our veins, it does not clot because their is a balance between the things that make blood clot and those that stop it from clotting. This is called homeostasis.

To explain, imagine a set of old fashioned kitchen scales used to weigh the ingredients for a cake. If the weight of the flour equals the metal weights, then the scale is balanced. If something is added to one dish or the other then the scales tip. The balance has been upset.

When we cut ourselves, the bleeding stops because there are things that make the blood more likely to clot.

If someone is given a 'blood thinning' medicine called warfarin (coumadin) then they bleed for longer if they get cut because the scales do not tip so much to the side for clotting.

Inherited conditions that make blood more likely to clot

Factor V Leiden

About 5% of the population have a gene that increases the amount of a substance called factor V Leiden. It is estimated that nearly a quarter of people who have a DVT carry this gene. There are 2 genetic types, heterozygote when only one gene is carried and homozygote when 2 genes are present.

Those people with the more common heterozygote form have a risk 7 times greater than the general population of having a DVT. Those with the homozygote form have perhaps an 80 times greater risk.

So in a lifetime one would expect 7 of 1000 people with the common form of factor V leiden to have a DVT but 8 in 100 ( 80 in 1000) with the less common homozygote form to have a DVT.

Prothrombin 20210A

About 2% of the population have a gene for this factor that increase the risk of DVT.

There are some rare genetic conditions that make the blood more likely to clot. These factors normally prevent clotting but when there is not enough in the body then clots are more likely to form:

Protein C (0.2% of the population), protein S (0.1%), Antithrombin III (0.02%)

Too much Factor VIII, IX and XI can increase the risk of clotting.

Acquired conditions

Things that happen to you can increase the chance of having a DVT. The table below shows the condition, how common it is (prevalence) and the relative risk of having a DVT. This means how many times more likely it is that you will get a DVT above the average of 1 in 1000 over a lifetime.

Peurperium is the first month after having had a baby.

Condition How Common (Prevalence) Relative Risk What percentage of all DVTs caused by this. How many people with the condition would get a DVT in their lifetime.  
Cancer 3% 7 10% 7 per 1000  
Surgery 4% 6 16% 6 per 1000  
Immobilisation 2% 11 15% 11 per 1000  
Pregnancy 5% 4 15% 4 per 1000  
Peurperium 1% 14 12% 14 per 1000  
Oral contraceptive pill 10% 5   5 per 1000  
Hormone Replacement   2      


The oral contraceptive pill and hormone replacement therapy can increase the chance of having a DVT.

I am on the pill, should I stop?

 Most people who take the pill are young, the actual risk may be much less than 5 per 1000. It could be as low as 1 in 10,000. Since the risk of a DVT  after pregnancy is 14 per 1000 then the risks of taking the pill and not being pregnant are probably worth it.

Are the 'low dose' pills better?

The companies that sell the pill think that the newest types are less likely to cause DVT. The scientific evidence suggests that all pills that contain less than 50 micrograms of oestrogen have much the same risk of causing a DVT.

I have factor V leiden and I want to take the pill as a contraceptive.

There is evidence to show that taking the pill and having a genetic tendency to form a clots could increase the chance of a DVT dramatically. About 35 of 1000 people with factor V leiden and taking the pill would develop a DVT.

I am in my 50s and I am taking hormone replacement therapy.

If 1000 women took HRT then only 2 would be likely to get a DVT. However since most women taking HRT are over 50 then the risk of a DVT is greater than those who are younger ( see chart below)


Deep Vein Thrombosis and Flying

Since the death, from Pulmonary embolism, of a young woman at Heathrow airport there has been a great deal of interest in deep vein thrombosis. An enquiry is currently being conducted by the World Health Organisation into the relationship between flying and DVT.

Travel related DVT is not new. In 1954 Homan described 2 deaths from PE in air passengers but also in car passengers. In 1988 Cruickshank coined the term 'economy class syndrome'.

Heathrow airport caters for about 50 million passengers per year. About 12 people per year die from a pulmonary embolus at Heathrow each year. There is evidence that DVT occurs in a large number of people after flying long distances.

About 5 people die from Pulmonary embolus for every million flights that are over 10,000 km in length. Shorter flights have a much lower chance of causing pulmonary embolus.

What are the risks of a DVT after a long haul flight?

This is not yet known, but if 5 people die from PE for every million flights over 10,000km then 500 probably developed a DVT.

If 2000 passengers travel over 10,000km by plane 1 will get a DVT.

What is my risk of a DVT?

It is not possible to tell any individual what their chances of getting a DVT are but the more risk factors you have the greater the chance.

So if you are on the pill, are 35 years old, have just broken your leg and are travelling from Europe to America then you have a high chance of developing a DVT.