What are protein C and protein S tests?

Protein C and protein S tests use a sample of your blood to measure how many of these proteins you have and how well they're working. Though they are often done together, these are separate tests. A protein C test measures protein C in your blood; a protein S test measures protein S.

Normally, proteins C and S 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.

However, if you have a protein C or S deficiency (when your body doesn't make enough of these proteins), your blood may clot too much. It may also clot even when you don't have an injury. These deficiencies can cause:

• Deep vein thrombosis (DVT), a condition where blood clots form in the veins in your legs or pelvis.
• Pulmonary embolism, a life-threatening condition where blood clots break loose and travel to your lungs.

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?

A health care provider may use protein C and protein S tests 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.
• Determine the type of protein C or S deficiency you may have.
• 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.

To determine whether you have a protein C or S deficiency, your provider will measure these proteins' activity. These are called protein C and protein S functional tests, and they can show whether one or both of these proteins are the cause of any clotting problems.

To determine the type of protein C or protein S deficiency, your provider will measure the quantity of these two proteins. If the functional tests show that these proteins aren't working as they should, your provider can see how many of them you have in your blood. If your body isn't making enough protein C and/or S, they might not be able to stop your blood from clotting.

There are several types of protein C and S deficiency, but generally they can be broken down into two categories:

• Those that are caused by a low quantity of protein C or S. In other words, your body doesn't make enough of these proteins.
• Those that are caused by dysfunctional proteins C or S. In other words, your body makes enough of these two proteins, but they don't work as they should.

Knowing the deficiency type can help your provider make important treatment decisions.

Why do I need protein C and protein S tests?

Your provider may need to test protein C and S if you've had:

• A blood clot before the age of 50 without a known reason.
• A blood clot in an unusual location, such as in the veins of your kidneys, liver, or brain.
• Several blood clots.
• Treatment for a clotting disorder, such as a protein C and/or S deficiency. Your provider may want to use the tests to see if your treatment is working.

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.

You may also need protein C and protein S tests if you have a condition or are taking certain medicines that can cause a protein C or S deficiency. For example, you may need these tests if you have liver or kidney disease or if you have had severe infections.

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 not to take blood thinners (medicine that helps prevent 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 and 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 or long car rides and/or airplane trips.

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

• Make suggestions for traveling such as walking around on the plane or stopping often to stretch your legs if you're driving.
• Prescribe medicines that may reduce your risk.
• Teach you how to recognize certain warning signs such as sudden swelling or pain in your leg, chest pain, or shortness of breath.