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.

Normal values are 60-150% inhibition.

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

A lack (deficiency) of protein S can lead to excess clotting. These clots tend to form in veins, not arteries.

A protein S deficiency may be inherited. It can also develop due to pregnancy or certain diseases, including disseminated intravascular coagulation (DIC), liver disease, warfarin (coumadin) use, chronic antibiotic use, and HIV infection.

There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

Clot events such as a clot in the lung (pulmonary embolism) reduce protein C and S levels, and their measurements may be misleading until the clot is treated.

Schafer A. Thrombotic disorders: Hypercoagulable states. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 182.