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Protein S is a normal substance in your 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
Your health care provider may tell you to stop taking certain medicines for a 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
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 S and protein C help control 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 S deficiency.
Sometimes this test is done to find the cause of 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:
- Disseminated intravascular coagulation (DIC)
- HIV infection
- Liver disease
- Long-term antibiotic use
- Warfarin (Coumadin) use
Protein S 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 from one person than another.
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)
A problem such as a blood clot in the lung (pulmonary embolism) may reduce protein C and S levels. Measurements may be misleading until the clot is treated.
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.