Definition of Protein S:

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.

How the test is performed:

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.

How to prepare for the test:

Special preparation is usually not necessary.

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:

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.