Protein C is a test that measures the amount of a protein in the blood that affects clotting.

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 let the blood flow again. 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.

Certain drugs can interfere with this test. Your doctor may ask you to stop taking these medications for a time before the test. Be sure to tell your doctor about all medications and supplements you are taking before having this 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.

Protein C and protein S help regulate blood clotting. A lack of these proteins may cause blood clots to form in veins.

Sometimes, the test is used to find the reason for repeated fetal loss in pregnant women.

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, and chemotherapy can cause low protein C levels.

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

Some medicines that prevent blood clots from forming (anticoagulants), such as warfarin (Coumadin), decrease protein C and protein S levels. Health care providers may have trouble interpreting protein C or S measurements in patients who are taking oral anticoagulants. Clot-related events, like clots in the lung (pulmonary embolism), reduce protein C and S levels.

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 ThrombolyticTherapy: Evidence-Based Guidelines. Chest. September 2004;126:401S-428S.

Feery WG. Genetic thrombophilia. Prim Care. September 2004;31:685-709.