Protein C and Protein S Tests
Reviewed
Also called: Protein C antigen, Protein S antigen, PC, PS, Free PS, Autoprothrombin IIA
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.
Specimen: Blood sample drawn from a vein in your arm.
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 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 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.
A health care professional will take a blood sample from a vein in your arm, using a small needle. 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
Normally, proteins C and S work together to prevent your blood from clotting too much.
- Lower than normal (or proteins not working well): 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. Limitation: Having an inherited deficiency doesn't mean that you'll develop a blood clot. Next step: If your provider thinks your protein C or S deficiency is inherited, you may need a genetic test to find out for sure.
- Higher than normal: Higher than normal levels of protein C or S are not known to cause any health problems. Limitation: If you have the test too soon, the results will not be accurate. Next step: If you have questions about your results or how to manage a protein C or S deficiency, talk with your provider.
- 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
Your provider may recommend treatment depending on your test results and health history. If you have questions about your results or how to manage a protein C or S deficiency, talk with your provider.
- 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
Source: U.S. National Library of Medicine (MedlinePlus). Practices and reference ranges may differ in your country.
U.S. National Library of Medicine: Protein C and Protein S Tests