Protein S (Free)

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. Protein S deficiency is associated with a higher risk of venous blood clots and may be inherited or acquired.

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 S deficiency may show up with vitamin K antagonist medicines (such as warfarin) or vitamin K deficiency, liver disease, certain cancers, surgery, trauma, and conditions that use up clotting factors. Protein S levels also drop normally during pregnancy.

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. Certain blood thinners can also affect protein S results, so your provider will consider any medicines you take when interpreting the test. The type of anticoagulant you take matters: vitamin K antagonists like warfarin lower protein S activity and antigen results, while heparins and direct oral anticoagulants (such as dabigatran, rivaroxaban, apixaban, or edoxaban) can falsely raise protein S activity readings but generally don't affect free protein S antigen testing.