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
A blood sample is needed.
How to Prepare for the Test
Certain medicines can change blood test results:
- Tell your health care provider about all the medicines you take.
- Your provider will tell you if you need to stop taking any medicines before you have this test. This may include drugs that prevent blood clots (blood thinners).
- Do not stop or change your medicines without talking to your doctor first.
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. Afterward, there may be some throbbing or slight bruising. This soon goes away.
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 helps control blood clotting. A lack of this protein or problem with the function of this protein may cause blood clots to form in veins.
The test is also used to screen relatives of persons who are known to have protein S deficiency.
Sometimes this test is done to find the cause of repeated miscarriages.
Normal values are 60 to 150% inhibition.
Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or may test different samples. Talk to your doctor about the meaning of your specific test results.
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:
- Disorder in which the proteins that control blood clotting become over active (disseminated intravascular coagulation)
- HIV infection
- Liver disease
- Long-term antibiotic use
- Warfarin (Coumadin) use
A problem such as a sudden blood clot in the lung may reduce the protein S level.
Protein S level rises 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 lightheaded
- 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, Anastasi J, eds. Hematology: Basic Principles and Practice. 6th ed. Philadelphia, PA: Elsevier Saunders; 2013:chap 142.
McPherson RA. Specific proteins. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 19.
Update Date 1/27/2015
Updated by: Yi-Bin Chen, MD, Leukemia/Bone Marrow Transplant Program, Massachusetts General Hospital, Boston, MA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.