WHO/BS/2023.5460 WHO 3rd International Standard for Protein S, Plasma

Protein S is a vitamin K co-factor for protein C, which is an inhibitor within the coagulation cascade. A lack of protein S can lead to thrombophilia; therefore, a protein S International Standard was required to enable diagnostic measurement of protein S levels. Protein S deficiency can exist as three types – type I, II & III – the type of which can be determined diagnostically by the three analytes – functional, free antigen & total antigen – associated with the protein S. In addition to diagnostic use, the protein S reference material is also used to measure protein S levels in some therapeutic products, virus inactivated fresh frozen plasma and some prothrombin complex concentrates.