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Protein S Antigen Blood Test

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The Protein S Antigen test measures the amount of protein S in the blood to determine if excessive blood clots are due to a hereditary or acquired deficiency.


Test Code:


CPT Code(s):







No fasting is required. Avoid warfarin (Coumadin®) therapy 2 weeks and heparin therapy 2 days prior to collection. Check with your physician before stopping any medications.

Test Results:

3-5 days. May take longer based on weather, holiday, or lab delays.

Walk-In Lab is prohibited from selling Quest tests to residents in the following states:AZ, NY, NJ, RI

Order this Protein S Antigen test to measure the amount of protein S in the blood to determine if excessive blood clots are due to a hereditary or acquired deficiency. Protein S is a protein in the blood that help regulate blood clotting. When a blood vessel is injured, the process known as hemostasis occurs. During this process, platelets (blood cells) work with proteins S and C (coagulation factors) to form a clot at the laceration to help stop blood loss until the blood vessel has healed. There must be a sufficient platelet count and coagulation factors, and each must function normally for a stable clot to form.

The primary function of protein S is to join with protein C to help regulate blood clotting by disabling certain coagulation factors (factors V and VIII) to slow down the blood clot formation. A low level of these proteins, or if they don't function properly, may lead to excessive blood clotting or an abnormal clot called deep vein thrombosis (DVT). A DVT may occur deep in the veins of the arms or legs and travel to the lungs, causing a life-threatening clot known as a pulmonary embolism (PE).

A deficiency or dysfunction of proteins S or C may be caused by an underlying health condition, such as:

  • Pregnancy
  • Liver disease
  • Kidney disease
  • Severe infections (HIV/AIDS)
  • Cancer
  • Warfarin (Coumadin®) use
  • In some instances, it may be an inherited condition 

Protein S has two forms: free and bound, but only the free protein S joins with protein C. Therefore, protein S deficiencies can be divided into three categories:

  1. Deficiency due to insufficient quantity
  2. Deficiency due to abnormal function
  3. Deficiency due to decreased amount of free protein S levels, though total protein S levels are normal

Individuals may order this test if they have experienced excessive blood clots, blood clots in unusual locations, or blood clots that developed due to unknown reasons. This test can also be used to determine if the reduction in activity is due to reduced production or dysfunction and to help diagnose a hereditary or acquired deficiency.

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