Protein S activity is a test to judge your risk of blood clots. Blood clots that form in the veins are dangerous. Protein S is a substance that controls the blood clotting mechanism and prevents uncontrolled clot formation. High levels and activity happen in some situations, including advanced age, but cause no health problems. Reduced activity puts you at risk for blood clots and related dangers.
What Is Protein S?
Protein S is a protein made by the liver. Together with other proteins, it reduces thrombin formation. Thrombin is an essential part of blood clotting.
Clot formation is necessary to preserve life. Without proper clotting, events like childbirth and injuries would cause immense blood loss and even death. But blood clotting must be controlled. The clotting factors that form clots are always present in the bloodstream. Without curbs, they would form clots within the blood vessels and block them.
Many proteins are involved in the formation and control of blood clots. Protein S activity is a vital part of normal blood clotting and its control. Protein S, protein C, anti-thrombin, and tissue factor pathway inhibitor (TFPI) restrict clotting and prevent abnormal clot formation.
Protein S is so named because it was first isolated and studied in Seattle. It was discovered in 1976, and new functions are still being discovered.
What Is Protein S Deficiency?
Protein S prevents excessive blood clotting and clot formation in the veins. Deficiency of this vital substance causes venous thromboembolism (VTE). Hereditary protein S deficiency is a genetic disease that runs in families. This is a very rare condition, affecting 0.03% to 0.13% of people.
If you have low protein S activity, you are at risk for deep vein thrombosis, pulmonary embolism, or both. Such clot formation can be provoked by:
- Pregnancy
- Surgery
- Hormonal birth control
- Hormone replacement therapy (HRT)
- Air travel
However, about half of clotting events are unprovoked. They happen without any of these provoking factors. The risk of clotting events increases after age 55.
Clotting Disorders Caused by Protein S Deficiency
Clotting of the blood is a delicate balance between clot-promoting factors and clot-preventing factors. A reduction or excess of any of them can cause bleeding disorders or clotting conditions.
Thrombosis. A blood clot is called a thrombus. A blood clot developing in a vein is called venous thrombosis. It is more likely when you are immobile for a long time — long flights, bed rest, hospitalization, etc. Other conditions provoking thrombosis are surgery, obesity, age over 60, a family tendency to clotting, and hormone therapy.
Deep vein thrombosis (DVT). Thrombosis in a vein, usually in the legs. The blood flow to the affected part is reduced. You will notice swelling, throbbing, or cramping in one leg. The painful area is warm, and the veins feel swollen. These symptoms can happen in your arm or belly also, depending on where the clot forms.
Embolism. An embolus is a blood clot that has broken off from a DVT. It travels along the veins to the right side of the heart and gets pumped into the pulmonary artery.
Pulmonary embolism. This life-threatening condition happens when a clot breaks off from a DVT and travels through the heart to the lungs. You may feel breathless suddenly and have sharp chest pain that worsens when breathing deep. Your heart rate may be fast. Pulmonary embolism is a life-threatening emergency.
Venous thromboembolism (VTE). This is the umbrella term for DVT and pulmonary embolism.
Protein S Activity Test
The role of protein S in keeping clotting under control is well-known, but measuring it is not simple. It exists in the blood in two forms, free and bound to other proteins. Measuring the absolute amount of protein S does not give dependable information about the risk of VTE.
Protein S activity is also a measure of its deficiency. Measuring protein S activity can detect almost all types of deficiencies. The protein S activity test measures the activity of this substance in the blood to prevent clotting.
If your physician asks for this test, tell them what medicines you are taking. Blood thinners, hormones, and some others can alter test results.
Protein S Activity Normal Range
Normal values for protein S vary a little among laboratories. Each laboratory will establish its range of normal values and mention it in the report sent to your physician. In general, normal values are 60% to 150% of clotting inhibition.
Protein S Activity Low. Low activity of protein S can cause abnormal clotting and VTE.
Protein S Activity High. High protein S activity is found in several people. Older people often have high levels. But this does not seem to cause any disease. Protein S is believed to act as a cofactor (helper) to tissue factor pathway inhibitor (TFPI) and other factors controlling clotting. Protein S by itself doesn't prevent clot formation. High levels of this substance don't cause bleeding.
Who Needs the Protein S Activity Test?
Your physician may ask for this test if you are at risk for clotting disease.
- You have an unexplained blood clot. That means a blood clot without any provoking factors.
- Your family has a history of VTE.
- You have relatives known to have protein S deficiency.
- You have had more than one miscarriage.
This test lets your physician know if you are at increased risk of abnormal blood clots. People from families with VTE who themselves have low protein S activity are at high risk. Their likelihood of venous thrombosis is five to ten times higher than family members with normal levels of protein S. People with familial protein S deficiency have a 0.7% risk per year of the first venous clot. The recurrence risk is 6% to 10% a year.
More people die from venous thromboembolism (VTE) than from road accidents, breast cancer, and HIV put together. Of course, not all VTE is caused by protein S deficiency, but if you are at risk for this deficiency, your physician will ask for a protein S activity test. Depending on the results, they will prescribe medicines to reduce your risk of abnormal clotting.