Protein S
A natural anticoagulant that works as the partner of protein C to switch clotting off. A low level (protein S deficiency), inherited or acquired, raises the risk of abnormal clots in the veins. Pregnancy and some hormones lower it, so timing and interpretation need care.
What it measures
Protein S is one of the body's natural anticoagulants and acts as the essential co-factor for protein C: together they break down activated clotting factors, keeping clot formation in check. Like protein C, protein S is made in the liver and depends on vitamin K. The test measures protein S activity (and sometimes free protein S) as a percentage of normal, as part of investigating a tendency to form clots (thrombophilia). A low protein S level shifts the balance toward clotting and raises the risk of venous thrombosis — clots in the deep leg veins or the lung. This can be inherited (a genetic protein S deficiency) or acquired. Importantly, several everyday factors lower protein S without any inherited problem: pregnancy, oestrogen-containing contraceptives or hormone therapy, warfarin, vitamin K deficiency, liver disease, and acute illness or a recent clot. This makes protein S one of the trickier thrombophilia tests to interpret, so it is measured away from these influences and confirmed by repeat testing before a genuine deficiency is diagnosed. It is interpreted alongside protein C and antithrombin.
What a high value can mean
- A normal or high protein S level is reassuring — it does not indicate a clotting problem.
- A high level has no adverse clinical significance.
What a low value can mean
- Inherited protein S deficiency — a genetic thrombophilia raising venous clot risk.
- Pregnancy — physiologically lowers protein S.
- Oestrogen — the pill or hormone therapy lowers it.
- Warfarin and vitamin K deficiency — protein S is vitamin-K-dependent.
- Liver disease, acute illness or a recent clot — can lower it.
When to discuss with a doctor
Protein S is a specialist test within a thrombophilia work-up, not a routine check. It is measured mainly after a blood clot that is unexplained, unusually severe, recurrent, in an unusual site or at a young age, or with a strong family history — to look for an inherited tendency to clot. Because pregnancy, oestrogen, warfarin, liver disease and a recent clot all lower protein S, a single low result does not by itself mean an inherited deficiency: the timing matters and the test is usually repeated away from these influences before any conclusion. A confirmed deficiency can influence treatment, prevention and advice for relatives. Mediora.AI shows protein S alongside protein C, antithrombin and D-dimer so the thrombophilia pattern is read together and reversible causes considered; confirming and acting on a low result belongs with your doctor or haematologist.