Platform
Company
Coagulation
Review status
Currently under review
Pending specialist review and validation.
This test measures how well protein C, a natural anticoagulant protein, is working in your blood. Protein C helps control clot formation by turning off parts of the clotting process when they are no longer needed.
A preliminary protein C activity test is typically used as a screening assay. If the screen suggests reduced function, your care team may order additional tests to confirm the finding and to determine whether the cause is inherited or acquired.
Too little working protein C can increase the tendency to form clots in veins, such as in the legs or lungs. Your clinician may order this test if you have had an unexplained blood clot, a strong family history of venous clots, skin changes after starting certain blood thinners, or if there is concern for conditions that can lower protein C.
Protein C levels and function can also be reduced by liver disease, vitamin K deficiency, severe infection, or while taking vitamin K antagonist medications. Understanding whether protein C function is low helps guide decisions about further testing, treatment, and prevention strategies.
A result lower than expected may reflect an inherited deficiency, an acquired condition that temporarily lowers protein C, or the effects of a medication. Because illness, recent clots, surgery, pregnancy, and certain drugs can affect results, your clinician may repeat testing when you are well and off interfering medicines before making a diagnosis.
If your result is in the expected range, protein C function is likely adequate, although it does not rule out other reasons for clotting. If your result is unexpectedly high, it is usually not clinically significant. Depending on your history, follow‑up may include a confirmatory activity test, antigen level, genetic testing, or evaluation for other clotting risk factors.
Reference intervals vary by laboratory, analyzer, methodology, population, and units. The ranges shown here are for education only. Always interpret your results against the reference interval printed on your own lab report.
Warfarin and related medicines lower protein C activity and can produce falsely low results. Testing is best timed after these drugs have been held and cleared, under your clinician’s guidance.
Some clot-based methods can be affected by apixaban, rivaroxaban, dabigatran, or edoxaban. Tell your care team about these medicines; the lab may use a method that minimizes interference or advise on timing.
During an acute thrombosis, severe infection, disseminated intravascular coagulation, or major inflammation, protein C can be consumed and appear low. Repeating the test after recovery provides a more reliable baseline.
Protein C is made in the liver and requires vitamin K. Liver dysfunction or vitamin K deficiency can lower protein C activity, which does not necessarily indicate an inherited deficiency.
Physiologic changes in pregnancy and the postpartum period, as well as estrogen-containing medications, can alter protein C results. Interpretation should consider these factors.
The test requires proper filling of a blue-top citrate tube and prompt processing. Underfilling, clotted or hemolyzed samples, or extreme hematocrit values can skew results.
Recent plasma transfusion or protein C concentrate can temporarily change results. Share any recent transfusion or factor replacement history with your clinician.
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