Protein C and protein S are two small proteins that
contribute to breaking down blood clots (thrombi). They are usually tested for
as part of an investigation into a possible tendency for blood clotting (hypercoagulable: meaning which coagulates, or clots, in an excessive manner) disorder, and/or to help diagnose the cause of a thrombotic episode or thromboembolism (the dislodging of a blood clot and travel to remote parts of the body),
mainly deep venous thrombosis (DVT)
or a venous thromboembolism(VTE),
especially in cases where it occurs in a relatively young person (less than 50
years old) or lodges in an unusual location, such as the veins leading to the
liver or kidney or the blood vessels of the brain (cerebral).

Tests
for protein C and protein S assess the function (activity)
or quantity (antigen)
of these proteins. Functional tests for protein C and protein S are usually
ordered, along with other tests for hypercoagulability, to screen for
sufficient, normal, factor activity. Based on those results, quantities of
protein C antigen and free, or sometimes total, protein S antigen may be
measured to check for decreased production which may be caused by an acquired
or inherited condition, and to classify the type of deficiency.

If
the patient has a family history of hypercoagulable disorders, then testing for
the levels and functions of protein C and S are valid when the patient is
exposed to situations that increase his or her risk of clotting, such as
surgery, chemotherapy for cancer, or oral contraceptive use.