(See also Overview of Thrombotic Disorders Overview of Thrombotic Disorders In healthy people, homeostatic balance exists between procoagulant (clotting) forces and anticoagulant and fibrinolytic forces. Numerous genetic, acquired, and environmental factors can tip... read more .)
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 Protein C Deficiency Because activated protein C degrades coagulation factors Va and VIIIa, deficiency of protein C predisposes to venous thrombosis. (See also Overview of Thrombotic Disorders.) Protein C is a vitamin... read more 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 in patients with disseminated intravascular coagulation Disseminated Intravascular Coagulation (DIC) Disseminated intravascular coagulation (DIC) involves abnormal, excessive generation of thrombin and fibrin in the circulating blood. During the process, increased platelet aggregation and coagulation... read more (DIC), liver disease, and vitamin K deficiency and in those on warfarin therapy. Protein S deficiency can also occur when patients use estrogen replacement therapy or contraception and during pregnancy due to the influence of estrogen on free protein S levels. Inflammation also influences free protein S levels. This phenomenon contributes to the hypercoagulability associated with these states.
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). Protein S activity can also be measured, but the assay is technically difficult and is associated with a high false-positive rate (10 to 15%) so more reproducible antigen assays are favored.
Treatment of Protein S Deficiency
The treatment of protein S deficiency associated with venous thrombosis is identical to the treatment of protein C deficiency Treatment Because activated protein C degrades coagulation factors Va and VIIIa, deficiency of protein C predisposes to venous thrombosis. (See also Overview of Thrombotic Disorders.) Protein C is a vitamin... read more , 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.
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