Protein C and Protein S Tests
Reviewed
These tests measure the levels and function of protein C and protein S in your blood. These proteins work together to prevent excessive blood clotting. Low levels or poor function can increase your risk of abnormal blood clots, such as deep vein thrombosis or pulmonary embolism. The tests help find the cause of unexplained blood clots, diagnose a protein C or S deficiency, and monitor treatment. Results should be interpreted by your healthcare provider, as reference ranges may vary.
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.
Understanding this test
Use the reference interval and units printed by the laboratory that performed the test. A result outside that interval is not a diagnosis by itself, and a result inside it does not rule out every condition. Your symptoms, history, medicines, timing, and related tests can change what the result means.
- 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.
Evidence source
U.S. National Library of Medicine: Protein C and Protein S Tests