It was discovered in 1996 that a specific change in the genetic code causes the body to produce too much of the prothrombin protein. Having too much prothrombin makes the blood more likely to clot. People with this condition are said to have a prothrombin mutation, also called the prothrombin variant, prothrombin G20210A, or a factor II mutation.

How Did I Get the Prothrombin 20210 Mutation?

You inherit 2 copies of all of your genes from your parents; one from your mother and one from your father. Therefore, we all have 2 prothrombin genes. It is possible to have a mutation in only 1 of your prothrombin genes. If this is the case, it is said that you are heterozygous for the gene mutation; you inherited the mutation from either your mother or your father. It is rare to have a mutation in both copies of the prothrombin gene, I.e., have inherited the mutation from both your mother and your father, but if you do, you are said to be homozygous.

How Is the Diagnosis Made?

The diagnosis of a prothrombin mutation is made by a blood test. The blood is sent to a laboratory for analysis of the DNA (genetic code), and this will reveal if you are homozygous or heterozygous. Even though the prothrombin mutation slightly increases factor II (prothrombin) levels, it is not helpful to determine blood levels of factor II when trying to determine whether a person has the mutation or not.

What Are the Implications of Having a Prothrombin 20210 Mutation?

Heterozygous prothrombin mutations are found in about 2% of the US white population. The mutation is uncommon in African Americans (approximately 0.5%) and is rare in Asians, Africans, and Native Americans. The homozygous form is considered uncommon, with an expected occurrence of approximately 1 in 10 000 individuals. The prothrombin 20210 mutation is equally as common in men and in women. It has nothing to do with blood type.

Having the prothrombin mutation increases the risk of developing a DVT (a blood clot in the deep veins, typically the legs) and/or PE (blood clot that travels to the lungs). DVTs are dangerous because they can damage the veins, leading to pain and swelling, and sometimes to disability. PEs can damage the blood vessels in the lung, leading to chest pain and shortness of breath, and are sometimes life-threatening. Approximately 1 in every 1000 people will develop a DVT or PE each year. The risk of developing a DVT or PE increases with age, with an average risk of 1 in 10 000 for people in their twenties to 1 in 200 for people in their seventies. Having a heterozygous prothrombin mutation increases the risk of developing a first DVT by about 2 to 3 times the background (or 2 to 3 in 1000 people each year). Having homozygous prothrombin mutations increases the risk further, but it is not yet known how much the risk is increased.

It should be noted that many people with the prothrombin mutation will never develop a blood clot in their lifetime. Very often, people who have the prothrombin mutation and develop a blood clot have additional risk factors.

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