Proceeds from website advertising help sustain Lab Tests Online. AACC is a not-for-profit organization and does not endorse non-AACC products and services.

Protein C and Protein S

Print this article
Share this page:
Also known as: Protein C Activity; Protein C Level; Protein S Activity; Protein S Level
Formal name: Protein C, Functional or Antigen; Protein S, Functional or Antigen (Free or Total)

Board approvedAll content on Lab Tests Online has been reviewed and approved by our Editorial Review Board.

The Test Sample

What is being tested?

Protein C and protein S are two proteins in the blood that help regulate blood clot formation. Two separate tests for these proteins are often performed together as part of the investigation of a possible excessive clotting disorder. The tests measure the amount of each protein and evaluate whether they are performing their proper functions.

Normally, when a body tissue or blood vessel wall is injured, a process called hemostasis begins to form a plug at the injury site to help stop the bleeding. Small cell fragments called platelets adhere to and aggregate at the site and a coagulation cascade begins as proteins called coagulation factors are activated one after the other. Eventually, a stable clot forms, preventing additional blood loss and remaining in place until the injured area has healed. The clot is then broken down when it is no longer needed. There must be an adequate amount of platelets and clotting factors and each must function normally in order for a stable clot to form.

Proteins C and protein S work together to help control blood clot formation. They inactivate specific coagulation factors (factors V and VIII) that are required to generate and form blood clots. This has the net effect of slowing down clot formation, much like brakes slow a speeding car. However, if there is not enough protein C or S or they are not functioning normally, clot formation can go unchecked, possibly leading to excessive clotting. These conditions can range from mild to severe.

Deficient or dysfunctional protein C or protein S may be due to an underlying condition (acquired), such as liver disease, kidney disease, severe infections or cancer, or can be inherited, passed from parent to child. About 1 out of every 300 people has one normal gene and one abnormal gene (heterozygous) for protein C deficiency and about 1 in 20,000 people have protein S or C deficiencies that lead to symptoms.

There are two types of inherited protein C deficiencies:

  • Type 1 is related to quantity.
  • Type 2 is related to abnormal function and is less common than Type 1.

Protein S exists in two forms in the blood: free and bound to another protein, but only the free protein S is available to combine with protein C. There are three types of inherited protein S deficiencies:

  • Type 1 deficiency is due to an insufficient quantity.
  • Type 2 is due to abnormal function.
  • Type 3 is due to low free protein S levels, though total protein S levels are normal.

Two types of tests may be used to evaluate protein C and protein S:

  • Functional tests for protein C and protein S measure their activity and evaluate their ability to regulate and slow blood clotting. Decreased activity may be due to a decreased amount of protein C or S or, more rarely, due to dysfunctional protein C or S.
  • Protein C and protein S antigen tests measure the amount of the protein present. Protein S antigen tests measure either free protein S or total protein S or both.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

You must wait at least 10 days after a thrombotic episode and be off oral warfarin (Coumadin®) anticoagulant therapy for 2 weeks before having this test done. (See also Common Questions #1.)