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Protein S (Blood)

What is this test?

This test measures levels of protein S, a protein in the blood that helps prevent blood clots. Protein S works along with another protein in the blood, called protein C, to help your blood clot normally.

If you don't have enough protein S in your blood, you have a condition called a protein S deficiency. This means that your blood may clot too much. Protein S deficiency is usually an inherited condition. You can inherit the abnormal (mutated) gene that reduces the level of protein S in the blood. The gene does this by affecting how much of the protein your body makes.

Protein S deficiency increases your risk for blood clots, including a serious condition called deep vein thrombosis (DVT). DVT causes dangerous blood clots to form in your arms or legs. These blood clots may travel throughout the body and settle in your lungs. A blood clot in the lungs (pulmonary embolism, or PE), can be life-threatening. Healthcare providers use the term venous thromboembolism (VTE) to describe the two conditions, DVT and PE. They use the term VTE because the two conditions are very closely related and because their prevention and treatment are closely related.

Protein S deficiency can be mild or severe. About 1 in 500 people will have a mild form of protein S deficiency. No one knows how many people have severe protein S deficiency, but the condition is thought to be quite rare. In severe forms of protein S deficiency, blood clots can form in small vessels throughout the body and can be life threatening.

Why do I need this test?

You may need this test if you have had a blood clot or VTE, including a DVT or a PE. You may also need this test if one of your parents has a protein S deficiency, since the condition can be inherited.

What other tests might I have along with this test?

Your healthcare provider may order other tests, including a protein C test. Proteins C and S work together to help the blood clot normally.

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your healthcare provider.

Levels of protein S in the blood can be affected by surgery, pregnancy, birth control pills, and hormone replacement therapy, in addition to other health conditions.

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

What might affect my test results?

Some medicines may affect your results. These include blood thinners (anticoagulants), birth control pills, and hormone replacement therapy.

How do I get ready for this test?

You don't need to prepare for this test. Your healthcare provider will tell you if you need to avoid eating or drinking in the hours before the test or skip any of your medicines on the day of the test.

Tell your provider if you are taking any medicines that can affect the way that your blood clots, such as warfarin. And be sure your provider knows about all other medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.


Medical Reviewers:

  • Calame, Rosemary, APRN, FNP, BC
  • Freeborn, Donna, PhD, CNM, FNP