Protein C 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.

Certain drugs can interfere with this test. Be sure to tell your doctor about all medications and supplements you are taking before having this test.

Some medicines that prevent blood clots from forming (anticoagulants), such as warfarin (Coumadin), decrease protein C and protein S levels. Your doctor may ask you to stop taking these medications for a time before the test.

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 C and protein S help regulate 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 C deficiency. It may also be done to find the reason for repeated miscarriages.

Normal values are 60% to 150% inhibition.

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

Protein C deficiency can be either passed down through families (inherited) or it can develop with other conditions. Liver disease, disseminated intravascular coagulation (DIC), warfarin (Coumadin), chronic antibiotic use, and chemotherapy can cause low protein C levels.

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)

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