Protein C and Protein S Tests

Protein C and protein S tests use a sample of your blood to measure how many of these proteins you have and how well they're working. Though they are often done together, these are separate tests. A protein C test measures protein C in your blood; a protein S test measures protein S.

Normally, proteins C and S work together to prevent your blood from clotting too much. Tests of protein C and S use a sample of your blood to measure how much of these proteins you have and how well they're working. However, if you have a protein C or S deficiency (when your body doesn't make enough of these proteins), your blood may clot too much.

It may also clot even when you don't have an injury. These deficiencies can cause: Deep vein thrombosis (DVT), a condition where blood clots form in the veins in your legs or pelvis.; Pulmonary embolism, a life-threatening condition where blood clots break loose and travel to your lungs.; Most protein C and protein S deficiencies are acquired. That means that the deficiency is caused by another health condition or certain medicines.

Acquired protein C and S deficiencies may develop with: Infections.; Liver or kidney disease.; A lack of vitamin K.; Cancer and chemotherapy.; Certain medicines, including birth control pills and warfarin, a blood-thinner.; Pregnancy.; Disseminated intravascular coagulopathy (DIC), a rare condition that causes blood clotting throughout the body, followed by bleeding.; In rare cases, protein C or protein S deficiencies are inherited. That means that your condition is caused by a change in a gene that one or both of your parents passed down to you. There are separate tests for protein C and S.

But they are often done at the same time.

Why this test is done

Your provider may need to test protein C and S if you've had:

• A blood clot before the age of 50 without a known reason.
• A blood clot in an unusual location, such as in the veins of your kidneys, liver, or brain.
• Several blood clots.
• Treatment for a clotting disorder, such as a protein C and/or S deficiency. Your provider may want to use the tests to see if your treatment is working.

In certain cases, you may need testing if you have a family member who has a serious inherited protein C or S deficiency or who had a clot at a young age. Testing may also be needed for a newborn with a clotting disorder.

You may also need protein C and protein S tests if you have a condition or are taking certain medicines that can cause a protein C or S deficiency. For example, you may need these tests if you have liver or kidney disease or if you have had severe infections.

How to prepare

If you are having protein C and S tests because you had a blood clot, you must wait until you have fully recovered to have this test. If you have the test too soon, the results will not be accurate. Your provider will let you know when you're ready to be tested.

Your provider may tell you not to take blood thinners (medicine that helps prevent blood clots) for several days or longer before your test. But never stop any prescription medicine without talking with your provider first.

Sample type

Blood

Other names for this test

protein C antigen, protein S antigen, PC, PS, free PS, autoprothrombin IIA