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Protein C is a normal substance in the body 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
How to Prepare for the Test
Certain drugs can interfere with this test. Be sure to tell your doctor about all medicines 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 medicines for a time before the test.
How the Test will Feel
You may feel slight pain or a sting when the needle is inserted to draw blood. You may feel some throbbing afterward.
Why the Test is Performed
You may need 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 or problem with the function 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 - 150% inhibition.
Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
The examples above show the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.
What Abnormal Results Mean
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 passed down through families (inherited) or it can develop with other conditions, such as:
- Chemotherapy use
- Disseminated intravascular coagulation (DIC)
- Liver disease
- Long-term antibiotic use
- Warfarin (Coumadin) use
Protein C levels rise with age, but this does not cause any health problems.
There is very little risk involved with having your blood taken. Veins and arteries vary in size so it may be harder to take a blood sample in one person than another.
Other slight risks from having blood drawn 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)
Anderson J, Weitz JI. Hypercoagulable states. In: Hoffman R, Benz EJ Jr, Silberstein LE, Heslop HE, Weitz JI, eds. Hematology: Basic Principles and Practice. 6th ed. Philadelphia, PA: Saunders Elsevier; 2012:chap 142.
Schafer A. Thrombotic disorders: Hypercoagulable states. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Saunders Elsevier; 2011:chap 179.
Reviewed By: Yi-Bin Chen, MD, Leukemia/Bone Marrow Transplant Program, Massachusetts General Hospital. Also reviewed by A.D.A.M. Health Solutions, Ebix, Inc., Editorial Team: David Zieve, MD, MHA, Bethanne Black, Stephanie Slon, and Nissi Wang.