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Protein S 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
Your health care provider may tell you to stop taking certain drugs for a certain amount of time before the test. Drugs called anticoagulants, such as warfarin (Coumadin), can decrease protein S levels. Health care providers may find it hard to interpret protein S measurements if you take this type of medicine.
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 S and protein C help control 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 S deficiency.
Sometimes this test is done to determine why a woman has 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 S can lead to excess clotting. These clots tend to form in veins, not arteries.
A protein S deficiency may be inherited. It can also develop due to pregnancy or certain diseases, including:
Protein S 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)
Clot events such as a clot in the lung (pulmonary embolism) reduce protein C and S levels, and their measurements may be misleading until the clot is treated.
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 on 2/28/2011
- 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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