What are protein C and protein S tests?

These tests measure the levels of protein C and protein S in your blood. Protein C and protein S tests are two separate tests that are often done at the same time.

Protein C and protein S work together to prevent your blood from clotting too much. Normally, your body makes blood clots to stop bleeding after a cut or other injury. If you don’t have enough protein C (protein C deficiency) or enough protein S (protein S deficiency), your blood can clot more than you need it to. If this happens, you may get a clot that partly or completely blocks blood flow in a vein or artery. These clots can form in the arms and legs and travel to your lungs. When a blood clot forms in the lungs it’s called a pulmonary embolism. This condition is life-threatening.

Protein C and protein S deficiencies can be mild or severe. Some people with mild deficiencies never have a dangerous blood clot. But certain factors can increase the risk. These include surgery, pregnancy, certain infections, and extended periods of inactivity, such as being on a long airline flight.

Protein C and protein S deficiencies are sometimes inherited (passed down from your parents), or can be acquired later in life. Testing may help find ways to prevent the formation of clots, regardless of how you got the deficiency.

Other names: protein C antigen, protein S antigen

What are they used for?

Protein C and protein S tests are used to diagnose clotting disorders. If tests show you have a protein C or a protein S deficiency, there are medicines and lifestyle changes you can make to reduce your risk of clots.

Why do I need protein C and protein S tests?

You may need these tests if you have certain risk factors. You may be at higher risk of a protein C or a protein S deficiency if you:

• Have a family member who has been diagnosed with a clotting disorder. Protein C and protein S deficiencies can be inherited.
• Had a blood clot that can’t be explained
• Had a blood clot in an unusual location such as the arms or the blood vessels of the brain
• Had a blood clot and are under the age of 50
• Had repeated miscarriages. Protein C and protein S deficiencies sometimes cause clotting problems that affect pregnancies.

What happens during protein C and protein S testing?

A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

Will I need to do anything to prepare for the test?

Your health care provider may tell you to avoid certain medicines for several days or longer before your test. Blood thinners, medicines that prevent clots, can affect your results.

Are there any risks to the test?

There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

What do the results mean?

If your results show low levels of protein C or protein S, you may be at risk of a dangerous clot. While there is no cure for protein C and protein S deficiencies, there are ways to reduce your risk of clots.

Your health care provider will make a treatment plan based on your results and health history. Your treatment may include medicines that make it harder for the blood to clot. These include blood thinning drugs called warfarin and heparin. Your provider may also recommend lifestyle changes, such as not smoking and not using birth control pills.

If you have questions about your results, talk to your health care provider.

Is there anything else I need to know about protein C and protein S tests?

If you have a family history or previous history of clotting, and are pregnant, be sure to tell your health care provider. Protein C and protein S deficiencies can cause dangerous clots during pregnancy. Your provider can recommend steps to ensure you and your baby stay healthy. These may include medicines, and/or frequent tests to monitor your condition.