Protein S
Protein S is a natural anticoagulant that works together with protein C to help regulate clotting. A blood test measures its level or activity.
Protein S helps keep clotting in check, so measuring it can help explain some tendencies to form clots. It is commonly checked during an evaluation for unexplained or repeated clots. It is interpreted alongside protein C and other clotting tests.
A high protein S level is generally not considered a health concern and rarely needs action. It is not typically associated with a specific problem. Your clinician will interpret it in the context of why the test was done.
A low protein S level can be linked to a greater tendency to form clots and may be inherited or acquired, including during pregnancy or with certain medications. Your clinician will use additional testing to interpret a low result.
A value within the normal range supports balanced regulation of clotting. Your clinician will interpret your result based on your personal and family clotting history.
Sources
Educational context only, not medical advice or a diagnosis. Reference ranges vary by lab, age and sex; always discuss your results with a clinician.