Prior to 1993 people with known blood clotting history, those that are termed with having hypercoagulable states , were only being tested for Protein C & S, Antithrombin III, and dysfibrinogenemias. The outcome of these test showed positive results approximately 25% of the time. This of course meant that 75% of the people were not being diagnosed. In Holland in 1993 some experiments were done to determine the cause of these undiagnosed cases. They did not find the mutation, however they did find the gene for factor five to be the cause of Activated Protein C Resistance. In 1994 the single point mutation on the factor five gene was found in Leiden Netherlands.

Factor five Leiden is now the most common hereditary coagulation disorder in the United States. It is found in 5% of Caucasians and 1.2% of the Afro-American population. FVL has its lowest frequencies amongst the Hispanic Americans, African Americans and almost never with indigenous people from Asia, America and Australia. The highest reports are seen in Europeans, some areas report as high as 15% of the population as carriers. FVL is autosomal dominant with inheritance. This means that it affects men and women equally and passes from generation to generation.

If you inherit one bad gene than you will be heterozygous , studies show the risk for venous thrombosis to be increased 3-8 fold in this case. If you inherit two bad genes than you would be homozygous and your risk for venous thrombosis could be 30-140 fold compared to a person without FVL.

Factor five is produced by the liver and circulates in the blood, generally inactive until there is a injury to a vessel wall, than specific proteins will be activated in response to the injury. Factor five is part of this response system known as the blood clotting cascade. One of the functions of FV is to work as a coagulant within this system to help with the formation of blood clots. It's second function is to work as a anticoagulant and help stop the formation of blood clotting so that the clot does not get too big or so too many clots do not form. This is where the problem is if you have FVL. Due to the mutation it causes Activated Protein C Resistance and the FV can stay in circulation longer, which can increase your risk of clotting more. The inactivation rate can be ten times slower than that of someone with normal FV.

Not everyone who gets blood clots will have FVL. There are many reason for any person to be at risk for clots, such as, fractures, hormone replacement therapy, surgery, birth control. Those with FVL can just have a greater risk.