What are protein C and protein S tests?

Protein C and protein S are proteins in your blood that work together to prevent your blood from clotting too much. Tests of protein C and S use a sample of your blood to measure how much of these proteins you have and how well they're working.

Problems with proteins C and S are called "deficiencies." Normally, your body makes blood clots to stop bleeding when you have an injury. If you have a protein C or a protein S deficiency, your blood may clot even when you don't have an injury.

The clots usually form in the veins in your legs or pelvis. This condition is called deep vein thrombosis (DVT). If part of a blood clot breaks loose and travels to your lungs, it's called a pulmonary embolism. This condition can be life-threatening.

Most protein C and protein S deficiencies are acquired. That means that the deficiency is caused by another health condition or certain medicines. Acquired protein C and S deficiencies may develop with:

• Infections
• Liver or kidney disease
• A lack of vitamin K
• Cancer and chemotherapy
• Certain medicines, including birth control pills and warfarin, a blood-thinner
• Pregnancy
• Disseminated intravascular coagulopathy (DIC), a rare condition that causes blood clotting throughout the body, followed by bleeding

In rare cases, protein C or protein S deficiencies are inherited. That means that your condition is caused by a change in a gene that one or both of your parents passed down to you.

There are separate tests for protein C and S. But they are often done at the same time.

Other names: protein C antigen, protein S antigen, PC, PS, free PS, autoprothrombin IIA

What are they used for?

Protein C and protein S tests are used to:

• Find the cause of a blood clot that can't be explained and check your risk for developing more blood clots in the future.
• Check for a protein C or S deficiency.
• Check your protein C or S levels to see if they've gotten better or worse. This is done if you have had a change in the health condition that is causing a protein C or S deficiency.

Why do I need protein C and protein S tests?

You may need to test protein C and S if you:

• Had a blood clot before the age of 50 without a known reason
• Had a blood clot in an unusual location, such as in the veins of your kidneys, liver, or brain
• Have had several blood clots

In certain cases, you may need testing if you have a family member who has a serious inherited protein C or S deficiency or who had a clot at a young age. Testing may also be needed for a newborn with a clotting disorder.

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?

If you are having protein C and S tests because you had a blood clot, you must wait until you have fully recovered to have this test. If you have the test too soon, the results will not be accurate. Your provider will let you know when you're ready to be tested.

Your provider may tell you to not to take blood thinners (medicine that helps prevents blood clots) for several days or longer before your test. But never stop any prescription medicine without talking with your provider first.

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?

Higher than normal levels of protein C or S are not known to cause any health problems.

Lower than normal levels of protein C or S, or proteins that aren't working well, mean that you have a higher risk of developing a blood clot. Your level of risk depends on how abnormal your test results are.

If your provider thinks your protein C or S deficiency is inherited, you may need a genetic test to find out for sure. Having an inherited deficiency doesn't mean that you'll develop a blood clot. But it does mean that your risk for developing a clot will be increased for the rest of your life.

If your protein C or S deficiency is acquired, the deficiency may be temporary or long-lasting. In certain cases, acquired deficiencies may get worse over time.

With both inherited and acquired deficiencies, treatment may help reduce your risk of developing blood clots. Your provider may recommend treatment depending on your test results and health history. You may be able to lower your risk of blood clots by:

• Taking medicines (blood thinners, also called "anticoagulants") to help prevent too much clotting
• Treating the condition that caused an acquired deficiency
• Avoiding other things that can increase your risk of blood clots, including:
  • Smoking
  • Birth control pills

If you have questions about your results or how to manage a protein C or S deficiency, talk with your provider.

Learn more about laboratory tests, reference ranges, and understanding results.

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

If you have been diagnosed with a protein C or S deficiency, it's important to talk with your provider whenever you face situations that can trigger blood clots, such as:

• Surgery
• Pregnancy
• Broken bones or other trauma injuries
• Not moving for long periods of time due to:
  • Bedrest
  • Long car rides or airplane trips

Your provider can help you reduce your risk of getting a clot during those higher-risk times.