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Protein S Functional Blood Test

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The Protein S Functional test measures protein activity to evaluate their ability to regulate blood clotting and screen for a potential excessive clotting disorder.


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Test Code:


CPT Code(s):


Also Known As:

Protein S Activity


Clot Detection


Blood, Plasma


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:

7-10 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 Functional test to measure protein S activity to determine if they properly regulate blood clotting and screen for a potential blood clotting disorder. Protein S and protein C work together to regulate the formation of blood clots. 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 properly for a stable clot to form.

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 in unusual locations or blood clots that developed due to unknown reasons. This test can also help determine if the decreased protein S activity is due to a deficiency or a rare dysfunction.

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