Your Clotting Brake Pedal
Protein C is one of your body's natural anticoagulants. When activated by thrombin, it degrades Factors Va and VIIIa, putting the brakes on clotting. It works with its cofactor Protein S. Without enough Protein C, your clotting system runs unchecked—increasing the risk of inappropriate blood clots.
What is Protein C?
Protein C is a vitamin K-dependent serine protease that, when activated by thrombin-thrombomodulin complex, inactivates Factors Va and VIIIa. It requires Protein S as a cofactor. Produced by the liver.
↑ What High Protein C Means
Not usually clinically significant.
Common symptoms:
Not clinically significant
↓ What Low Protein C Means
Weakened natural anticoagulant defense. Increased risk of DVT, PE, and other thrombotic events. Can be inherited or acquired (liver disease, vitamin K deficiency, warfarin, DIC).
Common symptoms:
Recurrent DVT or PE · Clots at young age · Warfarin-induced skin necrosis · Family history of clotting
Why It Matters
When normal:
Major natural anticoagulant pathway
Deficiency explains unexplained clotting events
Guides anticoagulation decisions
Warfarin caution: Protein C has shortest half-life of vitamin K-dependent factors
Risks if abnormal:
Low: 7-10x increased risk of venous thromboembolism
Warfarin-induced skin necrosis if Protein C drops before other factors
Inherited deficiency: autosomal dominant
What Can Cause Abnormal Levels?
Inherited Deficiency
30% likelyAutosomal dominant, prevalence ~1:500. Heterozygotes have 50% levels and moderate clot risk.
Liver Disease
40% likelyLiver produces Protein C. Disease impairs production.
Warfarin Therapy
Protein C is vitamin K-dependent. Warfarin lowers it. Short half-life means it drops first—paradoxical hypercoagulability early in warfarin therapy.
Vitamin K Deficiency
Vitamin K required for Protein C production.
DIC
Protein C is consumed.
Acute Thrombosis
Protein C is consumed at the clot site—don't test during acute event.
What You Can Do
Don't test during acute thrombosis, on warfarin, or with acute illness
Impact: All lower Protein C and give false results \u00B7 Timeline: Test when stable
If lifestyle changes aren't enough:
If inherited deficiency: avoid additional clot risk factors
Impact: No smoking, avoid prolonged immobility, caution with estrogen \u00B7 Timeline: Lifelong
Bridge with heparin when starting warfarin (Protein C drops first)
Impact: Prevents warfarin-induced skin necrosis \u00B7 Timeline: 5-7 days overlap
Recommended retest: Confirm with repeat testing off anticoagulation; test family members
Got your blood test report?
Upload your PDF and understand ALL your markers in 2 minutes. Plain language. Traffic light status. No medical jargon.
Analyze My Report — FreeFirst report is free. No credit card needed.