1627. Protein S (activity)

Protein S is a vitamin K-dependent glycoprotein that plays a key anticoagulant role, acting as a cofactor for activated protein C (aPC) in the inactivation of coagulation factors Va and VIIIa. Determining protein S activity allows for the assessment of thrombophilia risk, particularly hereditary thrombophilia, and is an important component of the evaluation of patients with thromboembolic complications.

Protein S is a plasma protein that belongs to the system of physiological anticoagulants (substances that prevent blood clotting).

Protein S is synthesized primarily in the liver, as well as in endothelial cells and megakaryocytes, with the participation of vitamin K. It circulates in plasma in two forms:

• free form (≈40%) – active and involved in the coagulation cascade;
• complement-bound C4b-binding protein (≈60%) – biologically inactive.

Only the free form of protein S has an anticoagulant effect, accelerating the destruction of activated coagulation factors V and VIII by protein C, leading to the suppression of thrombus formation.

Depending on the total and free protein S levels in the blood, three types of hereditary deficiency are distinguished:

• Type I – quantitative deficiency – reduced levels of total and free forms of protein S;
• Type II – qualitative deficiency – normal levels of total and free forms, but functional deficiency;
• Type III – decreased free form with normal total protein levels.

Types I and III are the most common and can be diagnosed by testing free protein S levels.

Decreased free protein S is observed in congenital deficiency, liver diseases with impaired synthetic function, nephrotic syndrome, disseminated intravascular coagulation, certain systemic diseases (lupus erythematosus), treatment with L-asparaginase, indirect anticoagulants (vitamin K antagonists), estrogen use (oral contraceptives), and during pregnancy and the postpartum period. Elevated free protein S levels are usually not associated with a medical problem and are considered clinically insignificant.

When?

• During a comprehensive diagnosis of thrombophilia.
• In the presence of episodes of arterial or venous thrombosis without obvious causes.
• To prevent recurrent thrombosis after an acute thrombotic event.
• During a comprehensive diagnosis of thrombotic complications during pregnancy.

Who should use it?

• Young people (under 50 years) with a familial history of thrombosis.
• Patients with recurrent thrombosis.
• Individuals with thromboembolic complications without other obvious causes.

• Venous blood

In order to exclude factors that may affect the test results, we recommend to follow the preparation rules: