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Protein S Deficiency
Because protein S helps activated protein C degrade clotting factors Va and VIIIa, deficiency of protein S predisposes to venous thrombosis.
Protein S, a vitamin K–dependent protein, is a cofactor for activated protein C–mediated cleavage of factors Va and VIIIa.
Heterozygous deficiency of plasma protein S predisposes to venous thrombosis (see Overview of Thrombotic Disorders). Heterozygous protein S deficiency is similar to heterozygous protein C deficiency in genetic transmission, prevalence, laboratory testing, treatment, and precautions (see Protein C Deficiency).
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, later, protein S) occur during disseminated intravascular coagulation and warfarin therapy.
Diagnosis is based on antigenic assays of total or free plasma protein S (free protein S is the form unbound to C4-binding protein).
The treatment of protein S deficiency associated with venous thrombosis is identical to the treatment of protein C deficiency (see Protein C Deficiency), with one exception. Because there is no purified protein S concentrate available for transfusion, only normal plasma can be used to replace protein S during a thrombotic emergency. It is not known if the newer oral anticoagulants that inhibit either thrombin (dabigatran) or factor Xa (eg, rivaroxaban, apixaban) can be used in place of other anticoagulants for this disorder.
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