Congenital thrombophilia

Factor V Leiden mutation = APC resistance

Thrombophilia is a condition in which the blood exhibits increased coagulability, tending to the occurrence of thrombosis.

What is the coagulation system?

Our coagulation system has various components. The blood contains certain proteins that are formed in the liver and enable the blood to coagulate, or clot. They are called clotting factors. When the body has been injured in some way, these factors step in and produce fibrin. Fibrin joins with thrombocytes (platelets) in the blood to form a thrombus, or plug (Fig. 1).

The coagulation system needs to be regulated as it would otherwise perpetually produce too many small thrombi. The clotting factors have what are termed antagonists: anti-coagulation agents that are able to cleave (split) certain of them. Following cleavage of the clotting factors by anticoagulants, they can no longer form fibrin. The system of clotting factors and their antagonists is in a delicate balance. If the clotting factors predominate or there is a deficit of inhibitors, the equilibrium shifts towards increased thrombus formation, with its consequent risk to the patient.

The make-up of clotting factors may be changed such that they are no longer cleaved by their antagonists, or are cleaved only slowly. This is what happens in factor V Leiden mutation.

Fig. 1