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Protein S (Blood)

Does this test have other names?


What is this test?

This test measures levels of protein S, a protein in the blood that helps it to clot. 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. It means that your blood may clot too much. That 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, known as a pulmonary embolism, can be life-threatening.

Protein S deficiency is an inherited condition. You can inherit the abnormal, or 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.

If you have a protein S deficiency, blood clots may form throughout your body and cause tissues to die because these clots block the circulation of blood. Protein S deficiency can be mild or severe. About one 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.

Why do I need this test?

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

What other tests might I have along with this test?

Your doctor 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 health care provider.

Levels of protein S in the blood are considered normal if they are between 65 and 140 percent. Levels that are higher than normal may mean that you have antibodies called lupus anticoagulants that make the blood clot. Levels that are lower than normal may mean that you have an inherited protein S deficiency.

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 medications, including anticoagulant medications, or blood thinners; oral contraceptives; and hormone replacement therapy medications may affect your test results.

How do I get ready for this test?

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

Tell your doctor if you are taking any medications that can affect the way that your blood clots, including Coumadin (warfarin) or aspirin. And be sure your doctor 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.