Protein S - Functional – BC260
🩸 Coagulation & Clotting TestReport: Next day Evening (Except Sunday)
Sample Type: Blood (Citrated Plasma)
Test Code: BC260
The Protein S - Functional test measures the activity of Protein S, a vitamin K-dependent protein that acts as the primary cofactor for activated Protein C. Together, Protein S and Protein C form a critical natural anticoagulant system that regulates blood clotting.
✔ A core component of a thrombophilia workup for patients with unexplained blood clots (thrombosis).
✔ Helps diagnose hereditary or acquired Protein S deficiency, a risk factor for abnormal clotting.
✔ Almost always ordered in conjunction with a Protein C test for a complete evaluation.
✔ Important to inform your doctor about any anticoagulant medications you are taking.
Protein S: The Essential Helper for Clot Prevention
While Protein C acts as the "brake" on the blood clotting cascade, **Protein S** acts as the "foot that presses the brake pedal." It is an essential cofactor, meaning activated Protein C cannot perform its anticoagulant function effectively without adequate, working Protein S. Therefore, a deficiency in Protein S has the same clinical consequence as a deficiency in Protein C: a weakened ability to regulate clotting, leading to a hypercoagulable state and an increased risk of thrombosis.
A **Protein S deficiency** can be inherited (passed down through families) or acquired due to conditions like pregnancy, liver disease, vitamin K deficiency, or certain medications. This functional test is crucial because it measures how well your Protein S is actually working, not just the total amount present in your blood.
Why a "Functional" Test is Critical for Protein S
Protein S exists in the blood in two forms: a "free" form that is active, and a "bound" form that is inactive. A simple antigen test might measure the total amount of Protein S (both free and bound) and give a normal result, even if the amount of active, free Protein S is dangerously low. The **functional test** specifically measures the activity of the free Protein S, giving a true picture of your body's ability to prevent clots and providing the most clinically relevant information to your doctor.
When is This Test Ordered?
A doctor will order a Protein S test alongside a Protein C test when investigating a potential clotting disorder (thrombophilia), particularly in individuals with:
- A history of unexplained or recurrent deep vein thrombosis (DVT) or pulmonary embolism (PE).
- A blood clot at a young age (typically before 50).
- A strong family history of thrombotic events.
- Blood clots in unusual locations, like the veins of the liver or intestines.
Frequently Asked Questions (FAQ)
They work together as a team. Protein C is a natural anticoagulant, and Protein S is its essential "helper" or cofactor. Protein C cannot function properly without Protein S. A deficiency in either one increases the risk of blood clots.
A low functional level indicates Protein S deficiency. This means your body's natural system for preventing excessive blood clots is impaired, which puts you at a higher risk for developing conditions like deep vein thrombosis (DVT).
Protein S exists in both active ("free") and inactive ("bound") forms in the blood. A functional test specifically measures the activity of the "free" form, which is what actually prevents clots. This gives a more accurate assessment of your clotting risk than a test that just measures the total amount.
Yes, Protein S levels naturally decrease during pregnancy. Therefore, testing for a hereditary deficiency is often best done before or a few months after pregnancy for the most accurate baseline results.
You must not stop any medication without consulting your doctor. Certain blood thinners, especially Vitamin K antagonists like warfarin, can affect Protein S levels. It is crucial that your doctor knows all the medications you are taking to interpret the results correctly.