What is a Free Protein S test?
The Free Protein S test measures the amount of free Protein S in the blood.
Protein S is a naturally occurring anticoagulant protein produced mainly by the liver. It works together with Protein C to help control blood clotting and prevent excessive clot formation.
Protein S circulates in the blood in two forms:
Free Protein S — the biologically active form.
Bound Protein S — attached mainly to a protein called C4b-binding protein.
Because free Protein S is the active form, measuring it can help evaluate whether the body's natural anticoagulant system is functioning properly.
What is the Free Protein S test used for?
The test may be used to:
Investigate an increased tendency to develop abnormal blood clots.
Help evaluate suspected Protein S deficiency.
Investigate unexplained deep vein thrombosis (DVT) or pulmonary embolism.
Assess patients with recurrent venous thromboembolism.
Support the investigation of thrombophilia, particularly in selected patients or families with a history of thrombosis.
Free Protein S is often evaluated together with Protein C activity and Antithrombin as part of a thrombophilia investigation.
Why do I need a Free Protein S test?
Your doctor may request this test if you have:
A blood clot at an unusually young age.
Recurrent venous blood clots.
A blood clot without an obvious cause.
A strong family history of abnormal blood clotting.
Thrombosis in an unusual location.
Other laboratory findings suggesting an inherited or acquired thrombophilia.
Testing may also be considered when a healthcare provider is investigating a possible inherited Protein S deficiency.
What happens during the test?
A healthcare professional will collect a blood sample from a vein in your arm using a small needle.
The laboratory separates the plasma from the blood cells and measures the concentration of free Protein S, usually using an immunological laboratory method.
Depending on the clinical situation, other Protein S measurements or coagulation tests may also be performed.
Will I need to do anything to prepare?
Usually, no fasting is required.
However, the result can be affected by several conditions and medications, including:
Pregnancy
Estrogen-containing contraceptives or hormone therapy
Vitamin K deficiency
Liver disease
Acute illness or inflammation
Anticoagulant medications, depending on the testing method
Recent or active thrombosis
Your healthcare provider may recommend testing at a specific time or repeating the test if an abnormal result is obtained.
Do not stop anticoagulant or other prescribed medication unless instructed by your healthcare provider.
Are there any risks?
The test requires a routine blood sample. You may experience mild discomfort, bruising, or slight bleeding at the needle site.
Serious complications are uncommon.
What do the results mean?
Free Protein S results are generally reported as a percentage, activity/concentration, or other laboratory-specific unit, depending on the assay.
The reference range depends on the laboratory method, age, and sex.
Low Free Protein S
A low Free Protein S level may indicate Protein S deficiency, which can increase the risk of venous blood clots.
Protein S deficiency may be:
Inherited (hereditary) — caused by genetic changes affecting Protein S.
Acquired — caused by other conditions or physiological states.
A reduced result can occur during pregnancy and with estrogen exposure, liver disease, vitamin K deficiency, acute thrombosis, or certain medications.
Therefore, one low result does not necessarily confirm inherited Protein S deficiency.
Normal Free Protein S
A normal result indicates that the measured free Protein S is within the laboratory's expected range.
A normal result generally makes significant Protein S deficiency less likely, but it does not exclude every possible cause of thrombosis.
The result should be interpreted together with the patient's clinical history and other thrombophilia investigations.
Important note about the results
Free Protein S testing can be affected by pregnancy, estrogen therapy, acute thrombosis, liver disease, vitamin K status, inflammation, and anticoagulant treatment.
If a low result is obtained, your doctor may recommend repeat testing when these influencing factors are no longer present before diagnosing an inherited Protein S deficiency.
For a complete assessment, Free Protein S may be evaluated together with Protein S activity, Protein S total antigen, Protein C, Antithrombin, Factor V Leiden, Prothrombin G20210A, and other tests when clinically indicated.
The diagnosis of Protein S deficiency should be based on appropriate laboratory testing together with the patient's personal and family history of thrombosis.
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