Protein S Activity

Normally, proteins C and S work together to prevent your blood from clotting too much.

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. Low protein S can be inherited or can develop later in life, and it is linked to a higher risk of clots forming in your veins.

Most protein C and protein S deficiencies are acquired. That means that the deficiency is caused by another health condition or certain medicines. Acquired causes can include vitamin K deficiency, blood thinners that work against vitamin K (such as warfarin), liver disease, certain cancers, severe clotting problems such as disseminated intravascular coagulation (DIC), surgery or major injury, and pregnancy. In newborns, the liver may not yet produce these proteins at adult levels, which can also lower the result.

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.

Some medicines can affect your test results. Blood thinners that work against vitamin K (such as warfarin) lower protein S activity, and pregnancy itself causes protein S to drop. Other anticoagulants can also interfere with the test in different ways: heparin generally has little effect at usual doses but may falsely raise activity at higher levels, and newer blood thinners such as direct thrombin inhibitors and factor Xa inhibitors can also falsely raise activity. Make sure your provider knows about all of the medicines you are taking and whether you are pregnant when interpreting your results.