Factor V Leiden is an autosomal dominant condition, where this coagulation factor has a mutation and cannot be destroyed by activated protein C (aPC). The actual point mutation causes an arginine to be replaced with a glutamine, at the 506th amino acid. This is a cleavage site for protein C.
As factor V cannot be inactivated, it continues to assist in the production of thrombin, and so thrombi form in the veins.
Up to 30% of patients who present with venous thrombosis, or pulmonary embolism have this mutation.
This disease can be diagnosed by watching the APTT (the time it takes for blood to clot) as activated protein C is added. With a normal patient, adding aPC increases the APTT. With Factor V Leiden, adding aPC will barely affect the time it takes for blood to clot.
There is also a simple genetic test that can be done for this disorder. The mutation (a 1691G→A substitution) removes a cleavage site of the restriction endonuclease MnlI, so simple PCR, treatment with MnlI, and then DNA electrophoresis will give a quick diagnosis.