Activated Protein C (APC) is a regulator of the coagulation cascade, by specifically inactivating factors Va and VIIIa, in the presence of phospholipids and calcium.

In most of the cases (more than 90%), Activated Protein C Resistance (APCR) phenotype is caused by a Factor V gene mutation (Factor V Leiden). The mutation, located on Factor V exon 10 (1691 G --> A), of arginine to glutamine on position 506, renders Factor Va resistant to the cleavage by Activated Protein C. This genetic anomaly can be evidenced with a clotting method performed in the presence or the absence of Activated Protein C.

Activated Protein C Resistance is tested by using a clotting method performed with or without Activated Protein C.