Protein C
Also known as: Protein C Activity
What Does Protein C Measure?
Protein C is a vitamin K-dependent anticoagulant protein produced by the liver that plays a critical role in regulating blood clotting. The Protein C test measures either the functional activity (how well Protein C works) or the quantity of Protein C present in the bloodstream. When activated by thrombin bound to thrombomodulin on endothelial cells, Protein C becomes Activated Protein C (APC), which inactivates clotting factors Va and VIIIa, thereby limiting the coagulation cascade and preventing excessive clot formation.
Why Does Protein C Matter?
Protein C deficiency—whether inherited or acquired—significantly increases the risk of abnormal blood clots (thrombosis), including deep vein thrombosis (DVT), pulmonary embolism (PE), and in severe cases, purpura fulminans in newborns. Testing Protein C levels is essential when evaluating unexplained or recurrent blood clots, especially in young individuals or those with a family history of clotting disorders. It also helps guide anticoagulation therapy decisions and monitor patients on warfarin, which can lower Protein C levels. Understanding a patient's Protein C status can be life-saving in preventing future thrombotic events.
Normal Ranges
Males
70–140% of normal activity (functional assay); approximately 3–5 µg/mL (antigen assay)
Females
70–140% of normal activity (functional assay); approximately 3–5 µg/mL (antigen assay)
Children
Varies by age; newborns typically 20–45%, gradually increasing to adult levels by adolescence
Causes of High Levels
- Diabetes mellitus (insulin resistance may upregulate hepatic Protein C synthesis)
- Nephrotic syndrome with protein loss leading to compensatory overproduction in some cases
- Use of certain hormone therapies or oral contraceptives (in some individuals)
- Hyperlipidemia (elevated cholesterol and triglycerides occasionally associated with elevated levels)
- Inflammatory states or acute phase reactions in some contexts
- Laboratory variability or sample timing relative to medications
Causes of Low Levels
- Hereditary Protein C deficiency (autosomal dominant mutation in the PROC gene)
- Liver disease (cirrhosis, hepatitis) reducing hepatic synthesis
- Vitamin K deficiency reducing carboxylation and functional activity
How to Improve Your Protein C
Diet
- Ensure adequate vitamin K intake through leafy green vegetables (spinach, kale, broccoli) to support Protein C carboxylation and function
- Consume a liver-supportive diet rich in antioxidants, including berries, cruciferous vegetables, and healthy fats to support hepatic Protein C synthesis
- Limit excessive alcohol consumption, which can impair liver function and reduce Protein C production
- Eat adequate dietary protein (lean meats, legumes, dairy) to provide amino acid building blocks for Protein C synthesis
- Maintain a balanced diet low in processed foods to reduce systemic inflammation that can impair coagulation regulation
Supplements
- Vitamin K2 (MK-7 form): 90–120 mcg/day to support vitamin K-dependent protein carboxylation (consult physician if on warfarin)
- Milk thistle (silymarin): 140–420 mg/day to support liver health and hepatic protein synthesis
- Omega-3 fatty acids: 1,000–2,000 mg/day EPA+DHA to reduce inflammation and support vascular health
- B-complex vitamins, particularly B12 (500–1,000 mcg) and folate (400–800 mcg), to support liver function and coagulation protein metabolism
Related Biomarkers
Frequently Asked Questions
What is Protein C deficiency and how dangerous is it?
Protein C deficiency is a condition where the blood has insufficient levels or function of Protein C, the natural anticoagulant that prevents excessive clotting. It significantly raises the risk of deep vein thrombosis (DVT), pulmonary embolism (PE), and stroke. Heterozygous deficiency (one abnormal gene) increases clot risk 5–10 fold, while homozygous deficiency (two abnormal genes) is rare but life-threatening, causing purpura fulminans in newborns. With proper anticoagulation management, most people with Protein C deficiency live normal lives.
Can warfarin cause low Protein C levels?
Yes, warfarin (Coumadin) reduces Protein C levels because it blocks vitamin K-dependent carboxylation, which is required for Protein C to become functionally active. In fact, when warfarin therapy is initiated, Protein C levels drop faster than other clotting factors, which can briefly create a paradoxical pro-thrombotic state known as warfarin-induced skin necrosis. This is why heparin is often given concurrently when starting warfarin, especially in patients known to have Protein C deficiency.
Should I be tested for Protein C deficiency?
Testing is recommended if you have had unexplained blood clots (especially at a young age), recurrent DVT or pulmonary embolism, a family history of clotting disorders, blood clots in unusual locations (e.g., mesenteric or cerebral veins), or a first-degree relative diagnosed with Protein C deficiency. Testing should ideally be done when you are not acutely ill, not pregnant, and not taking anticoagulants like warfarin, as these factors can artificially lower levels and give misleading results.