Protein C and Protein S

Other names: Protein S Free (Functional) or Antigen (Total); Protein C Functional or Antigen

Proteins C and S play an important role in the regulation of the rate of blood clotting; blood clotting is brought about by the activation of the coagulation cascade, which is a process that involves the activation of more than 20 different types of protein. Thrombin is a clotting factor, which helps to control the rate of clot formation; it forms a feedback loop, which contains proteins C and S to regulate and slow down the coagulation cascade.

Problems with protein S and C may either be inherited or acquired; there are two types of protein C deficiencies and 3 types of protein S deficiencies.

When is the test used?

The test may be used to assess the quantity or activity of protein C and S; the test is commonly used to investigate the cause of a venous thromboembolism (especially if the patient is younger than 50 or the clot has developed in an unusual place). Tests may also be used to assess factor activity; this test will usually be carried out alongside a number of other tests.

The test is usually ordered when a patient has had a thrombotic episode; the test should be done a minimum of ten days after the episode and the test should not be done while the patient is on anticoagulant therapy. Other tests may be carried out to see if the patient has another health condition, which may be affecting blood clotting.

The test may also be used after a thrombotic episode to determine the risk of the patient suffering another episode.

How is the test performed?

The test is done by collecting and analysing a sample of the patient’s blood; a needle is inserted into the vein (usually on the inside of the elbow) and the blood is collected in a syringe. When the doctor (or nurse) has a sufficient sample, the blood will be bottled, labelled and sent to the laboratory for testing.

What do the test results show?

If the test results are normal, this usually indicates that the clotting process is working effectively.

If levels of protein C and S are higher than normal, this is not normally a cause for concern as long and the results show that the proteins are functional and that the protein S is free. If the results show that the protein is not functioning normally, further tests will usually be ordered and the patient’s condition will be monitored closely.

Decreased levels of protein C and S may be caused by a number of health conditions, including:

  • Liver disease
  • Kidney disease
  • Severe infections
  • Vitamin K deficiency
  • Certain forms of cancer
  • HIV
  • Inflammatory conditions

Levels may also decrease during pregnancy or if the patient is receiving heparin anticoagulant therapy.

Specific Blood Tests

Blood Test Guide

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