Protein S, Functional Unit Converter

Protein S is a natural anticoagulant in your blood that helps prevent excessive clotting. It works by acting as a cofactor for activated protein C (APC), which inactivates clotting factors Va and VIIIa to slow down the clotting process.

About 40% of protein S circulates freely in the blood, while the remaining 60% is bound to a complement protein called C4b-binding protein. Only the free form is active as an anticoagulant.

The functional protein S test measures how well protein S is working in the clotting system, rather than simply measuring how much is present. This makes it useful for detecting all types of protein S deficiency, including rare cases where protein S levels are normal but the protein does not function properly.

Protein S is produced primarily by the liver. It is a vitamin K-dependent protein, meaning the body requires adequate vitamin K to synthesize it properly.