Protein C and Protein S Tests

Why this test may be ordered

• To find the cause of a blood clot that cannot be explained
• To check your risk for developing more blood clots
• To diagnose a protein C or S deficiency
• To determine the type of deficiency (low quantity or dysfunctional proteins)
• To monitor levels if you have a condition or are taking medicines that can cause a deficiency

Preparation and collection

If you are being tested because you had a blood clot, you must wait until you have fully recovered before having this test. Your provider may ask you to stop taking blood thinners (anticoagulants) for several days or longer before the test. Do not stop any prescription medicine without talking with your provider first.

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.

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.

Understanding results

Your test results will show the levels and activity of protein C and protein S. The following patterns describe what your results may mean. Always discuss your results with your healthcare provider.

• Normal levels: Your protein C and protein S levels are within the normal range. This means you do not have a deficiency that increases your risk of blood clots. Limitation: Normal results do not rule out all clotting disorders. Your provider will consider your overall health and history. Next step: No specific follow-up is needed based on this result alone. Continue routine care.
• Lower than normal levels: Your protein C and/or protein S levels are lower than normal, indicating a deficiency. This increases your risk of developing blood clots. The deficiency may be acquired (due to another condition or medication) or inherited. Your level of risk depends on how abnormal your test results are. Limitation: This test alone cannot determine if the deficiency is inherited. If your provider suspects an inherited deficiency, you may need a genetic test to confirm. Having an inherited deficiency does not mean you will develop a blood clot, but your risk is increased for life. Next step: Contact your healthcare provider to discuss your risk of blood clots and possible treatment. Treatment may include blood thinners, treating the underlying cause, and avoiding other risk factors such as smoking and birth control pills.
• Higher than normal levels: Higher than normal levels of protein C or S are not known to cause any health problems. Limitation: No known health risks are associated with elevated levels. Next step: No specific follow-up is needed based on this result alone. Continue routine care.

• If you have the test too soon after a blood clot, the results may not be accurate. You must wait until you have fully recovered.
• Certain medicines (including blood thinners, birth control pills, and warfarin) and health conditions (such as liver disease, kidney disease, infections, cancer, and pregnancy) can affect your protein C and S levels.
• Reference ranges may vary between laboratories. Your healthcare provider will interpret your results based on your specific situation.

What to do next

If you have been diagnosed with a protein C or S deficiency, talk with your provider whenever you face situations that can trigger blood clots, such as surgery, pregnancy, broken bones, or long periods of inactivity. Your provider can help you reduce your risk during these higher-risk times.

• What do my protein C and protein S test results mean for my health?
• Do I need any additional tests, such as genetic testing?
• What can I do to lower my risk of blood clots?
• Should I avoid any medicines or activities?
• How often should I have these tests repeated?

Evidence source

U.S. National Library of Medicine: Protein C and Protein S Tests