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