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.
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:
Protein C levels rise with age, but this does not cause any health problems.
What the risks are
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)
Bauer KB. Hypercoagulable states. In: Hoffman R, Benz EJ Jr., Shattil SJ, et al, eds. Hoffman Hematology: Basic Principles and Practice. 5th ed. Philadelphia, Pa: Churchill Livingstone Elsevier; 2008:chap 134.
Schafer A. Thrombotic disorders: Hypercoagulable states. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 182.
Last reviewed 2/28/2011 by David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; and Yi-Bin Chen, MD, Leukemia/Bone Marrow Transplant Program, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.
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