(See also Overview of Thrombotic Disorders.)
Protein S, a vitamin K–dependent protein, is a cofactor for activated protein C–mediated cleavage of factors Va and VIIIa. Protein S and protein C are, therefore, components of a natural plasma anticoagulant system.
Heterozygous deficiency of plasma protein S predisposes to venous thrombosis. Heterozygous protein S deficiency is similar to heterozygous protein C deficiency in genetic transmission, prevalence, laboratory testing, treatment, and precautions.
Homozygous deficiency of protein S can cause neonatal purpura fulminans that is clinically indistinguishable from that caused by homozygous deficiency of protein C.
Acquired deficiencies of protein C (and, soon thereafter, protein S) occur during disseminated intravascular coagulation (DIC) and warfarin therapy.
Diagnosis is based on antigenic assays of total or free plasma protein S (free protein S is the form unbound to the protein S carrier molecule, C4-binding protein).
The treatment of protein S deficiency associated with venous thrombosis is identical to the treatment of protein C deficiency, with one exception. Because there is no purified protein S concentrate available for transfusion, normal plasma is used to replace protein S during a thrombotic emergency.
It is probable, but not yet certain, that the direct oral anticoagulant (DOAC) inhibitors of either thrombin (dabigatran) or factor Xa (eg, rivaroxaban, apixaban) can be used in place of other anticoagulants for this disorder.