Protein C
Also called: Protein C activity
A natural anticoagulant — a vitamin-K-dependent protein that helps switch clotting off. A low level (protein C deficiency), inherited or acquired, raises the risk of abnormal clots in the veins, so it is checked as part of investigating an unexplained or unusual clotting tendency.
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What does Protein C measure?
Protein C is one of the body's natural anticoagulants — proteins that keep blood clotting in balance by switching it off once a clot has done its job. When activated, protein C (working with its partner protein S) breaks down two clotting factors (factors Va and VIIIa), damping the cascade. Like the PT clotting factors, protein C is made in the liver and depends on vitamin K. The test measures protein C activity as a percentage of normal, as part of investigating a tendency to clot (thrombophilia). A low protein C 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 C deficiency) or acquired — from liver disease, vitamin K deficiency, warfarin (which lowers protein C), or being consumed in a large clot or DIC. A well-known caution is that starting warfarin drops protein C quickly, briefly increasing clotting risk, which is why heparin overlaps warfarin at the start of treatment. Because warfarin, an acute clot and liver problems all lower protein C, the test is best done away from these, and interpreted with protein S and antithrombin.
What does a high Protein C level mean?
- A normal or high protein C level is reassuring — it does not indicate a clotting problem.
- A high level has no adverse clinical significance.
What does a low Protein C level mean?
- Inherited protein C deficiency — a genetic thrombophilia raising venous clot risk.
- Warfarin — lowers protein C (so the test is done off warfarin).
- Vitamin K deficiency — protein C is vitamin-K-dependent.
- Liver disease — less protein C is made.
- Active clotting or DIC — protein C is consumed.
When to discuss with a doctor
Protein C 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 when there is a strong family history — to look for an inherited tendency to clot. A confirmed deficiency can influence how a clot is treated and prevented and prompts consideration for relatives. Timing matters: because warfarin, a recent clot and liver disease all lower protein C, the test and its interpretation are handled carefully by a haematologist so a temporary dip isn't mistaken for an inherited deficiency. Mediora.AI shows protein C alongside protein S, antithrombin and the clotting times so the thrombophilia pattern is read together; confirming and acting on a low result belongs with your doctor or haematologist.
Related markers
Commonly confused with
- Protein S They differ by one letter and work as a pair: protein S helps protein C switch off clotting. A deficiency of either raises clotting risk, which is why they are almost always ordered together.