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.

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

Your health care provider may tell you to temporarily stop taking certain drugs before the test. Certain drugs called anticoagulants, such as warfarin (Coumadin), can decrease protein S levels. Health care providers may find it difficult to read protein S measurements if you take such medicine.

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.

Your doctor may order this test if you have an unexplained blood clot. 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.

Review Date: 03/02/2009
Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; and Yi-Bin Chen, MD, Leukemia/Bone Marrow Transplant Program, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org)