Definition of Protein C:

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.

How the test is performed:

A blood sample is needed. For information on how this is done, see: Venipuncture

How to prepare for the test:

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.

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:

Your doctor may order this test if you have an unexplained blood clot, or a family history of blood clots. 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.