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?

Test results may vary depending on your age, gender, health history, the method used for the test, and other things. Your test results may not mean you have a problem. Ask your healthcare provider what your test results mean for you. 

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 is done with a blood sample. A needle is used to draw blood from a vein in your arm or hand. 

Does this test pose any risks?

Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be 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 healthcare provider if you are taking any medicines that can affect the way that your blood clots, such as warfarin. Be sure your healthcare 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.