Interference by Activated Protein C Resistance in the Determination of Protein S Activity

Protein S is an anticoagulant protein and a cofactor of protein C. It is recommended to explore it first by a functional (chronometric) technique by assessing its cofactor activity of activated protein C. Existing tests measure the prolongation of a coagulation time. These tests are all dependent on factor V, but some patients have mutations in factor V (including V Leiden) that make them resistant to activated protein C activity. It is suspected that these mutations lead to underestimations of protein S activity by chronometric technique.