Definition of Protein S:

Protein S is a substance that prevents blood clotting. A blood test can be done to see how much of this protein you have in your blood.

How the test is performed:

A blood sample is needed. For information on how this is done, see: Venipuncture

How to prepare for the test:

Your health care provider may tell you to stop taking certain drugs for a certain amount of time before the test. Drugs called anticoagulants, such as warfarin (Coumadin), can decrease protein S levels. Health care providers may find it hard to interpret protein S measurements if you take this type of medicine.

How the test will feel:

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed:

Your doctor may order this test if you have an unexplained blood clot, or a family history of blood clots. Protein S and protein C help control blood clotting. A lack of these proteins may cause blood clots to form in veins.

The test is also used to screen relatives of patients with a known protein S deficiency.

Sometimes this test is done to determine why a woman has repeated miscarriages.