Factor V, genetic mutation test

Clinical significance
Factor V carrying the G1691A mutation (also called factor V Leiden, after the Dutch city where the mutation was discovered) is not inactivated by activated protein C, resulting in increased production of thrombin which, as its name suggests, promotes clotting and therefore the formation of thrombi.
It is inherited in an autosomal dominant pattern: carriers have a one in two chance of passing the predisposition on to their children.
The G1691A genetic variant (also called R506Q, from the position of the substituted amino acid) has a gene frequency of 1.4-4.2% in Europe, decreasing from north to south.
It is present in about 15% of unselected patients with venous thrombosis, and in 20-60% of people with thrombosis or selected because of family history.
In Italy the frequency of heterozygous carriers is 2-3%, while homozygotes account for 0.02%.
Heterozygotes have a 5-10 times higher risk than the general population of developing venous thrombosis, while homozygotes have a 50-100 times higher risk. In heterozygotes the risk of heart attack appears to be increased 2-3 times.