Protein C

Protein C is a natural anticoagulant protein made in your liver. It helps keep blood from clotting too easily by turning off clotting factors when they are no longer needed. Protein C works closely with protein S and the body’s endothelial system to regulate clot formation and breakdown.

Laboratories can measure Protein C in different ways. Some tests assess how well it functions (activity), while others measure how much of the protein is present (antigen). Your clinician may choose one or both types depending on your clinical situation.

If Protein C is too low or does not work properly, you may be at higher risk for forming venous blood clots, such as deep vein thrombosis or pulmonary embolism. Low levels can be inherited or acquired due to conditions like liver disease, severe infection, vitamin K deficiency, or the effects of certain medications. Testing is often ordered when there is an unexplained or early-onset clot, a strong family history of clots, or to evaluate causes of abnormal clotting.

Results can also help guide timing of treatment decisions and family counseling. Because Protein C can be temporarily reduced during acute illness or while taking some anticoagulants, clinicians often plan testing thoughtfully to ensure results reflect your baseline status.

Your report may include a functional (activity) result and/or an antigen level. A lower-than-expected activity result suggests reduced function, which can be due to inherited changes, liver or vitamin K issues, or consumption during severe illness. An antigen result helps distinguish whether there is too little protein made or whether the protein is present but not working well. Higher-than-expected results are uncommon and are usually less clinically significant, but your clinician will interpret them in the context of your history and other tests.

If results are abnormal, your clinician may confirm them with repeat testing when you are well and not on interfering medications. Sometimes family testing or genetic consultation is considered. Do not stop or change any medication on your own; discuss next steps with your care team, who will consider your symptoms, other lab results, and personal and family history.