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

Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to swell with blood.

A needle is inserted into the vein, and the blood is collected in an airtight vial or a syringe. During the procedure, the band is removed to restore normal blood flow. Once the blood has been collected, the needle is removed. The puncture site is covered to stop any bleeding.

For an infant or young child, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a small glass tube (pipette), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if bleeding continues.

Special preparation is usually not necessary.

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.

Protein S and protein C help control blood clotting. Lower-than-normal amounts of these proteins may cause blood clots to form, usually in veins.

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

Normal values are 60-150% inhibition.

Lower-than-normal protein S can result in too much blood clotting. These are usually vein clots, such as deep vein thrombosis (DVT).

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

• Excessive bleeding
• Fainting or feeling light-headed
• Blood collecting under the skin (hematoma)
• Infection
• Many needle sticks to find veins

Some drugs that stop clots from forming (anticoagulants), such as warfarin (Coumadin) can decrease protein C and protein S levels. Health care providers may find it difficult to read protein C or S measurements in patients taking anticoagulants.

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.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Getting a blood sample from some people may be harder than from others.

Büller HR, Agnelli G, Hull RD, Hyers TM, Prins MH, Raskob GE. The Seventh ACCP Conference on Antithrombotic and Thrombolytic Therapy: Evidence-Based Guidelines. Chest. September 2004;126(suppl):401S-428S.

Soliman DE, Broadman LM. Coagulation defects.Anesthesiol Clin North America. September 2006;24:549-578.