What is a Functional Protein C test?
The Functional Protein C test measures the activity of Protein C, a naturally occurring anticoagulant protein that helps control blood clotting.
Protein C is produced mainly by the liver and works together with Protein S to inactivate certain clotting factors, particularly Factor Va and Factor VIIIa. This helps prevent excessive formation of blood clots.
Unlike a Protein C antigen test, which measures the amount of Protein C present, the Functional Protein C test evaluates how well the Protein C is working.
What is the Functional Protein C test used for?
The test may be used to:
Investigate an increased tendency to develop abnormal blood clots.
Help diagnose Protein C deficiency.
Evaluate unexplained or recurrent venous thromboembolism.
Investigate deep vein thrombosis (DVT) or pulmonary embolism.
Assess selected patients with a strong family history of thrombosis.
Support the investigation of inherited or acquired thrombophilia.
Functional Protein C is often evaluated together with Free Protein S and Antithrombin as part of a thrombophilia investigation.
Why do I need a Functional Protein C test?
Your doctor may request this test if you have:
A blood clot at an unusually young age.
Recurrent venous blood clots.
A blood clot without an obvious cause.
A strong family history of abnormal blood clotting.
Thrombosis occurring in an unusual location.
Other findings suggesting a possible inherited or acquired thrombophilia.
What happens during the test?
A healthcare professional will collect a blood sample from a vein in your arm using a small needle.
The laboratory separates the plasma from the blood cells and measures the functional activity of Protein C using a specialized coagulation assay.
The result is commonly reported as a percentage of normal activity, although reporting units and reference ranges depend on the laboratory method.
Will I need to do anything to prepare?
Usually, no fasting is required.
However, Protein C activity can be affected by several conditions and medications, including:
Vitamin K deficiency
Liver disease
Acute illness or inflammation
Recent or active thrombosis
Certain anticoagulant medications
Some inherited or acquired conditions
Warfarin and other vitamin K antagonists can significantly reduce Protein C activity. Some direct oral anticoagulants may also interfere with certain Protein C assays.
Tell your healthcare provider about all medications you are taking. Do not stop anticoagulant treatment unless specifically instructed to do so.
Are there any risks?
The test requires a routine blood sample. You may experience mild discomfort, bruising, or slight bleeding at the needle site.
Serious complications are uncommon.
What do the results mean?
Functional Protein C results are generally reported as a percentage of normal activity.
Reference ranges vary according to the laboratory method, age, and other patient factors.
Low Functional Protein C
A low Protein C activity result may indicate Protein C deficiency, which can increase the risk of venous blood clots.
Protein C deficiency may be:
Inherited (hereditary) — caused by genetic changes affecting Protein C.
Acquired — associated with conditions such as severe liver disease, vitamin K deficiency, or certain medications.
A low result does not automatically confirm inherited Protein C deficiency.
If the result is unexpectedly low, your healthcare provider may recommend repeat testing and additional investigations after temporary or acquired causes have been considered.
Normal Functional Protein C
A normal Protein C activity result indicates that Protein C function is within the laboratory's expected range.
This generally makes significant Protein C deficiency less likely, although a normal result does not exclude every possible cause of abnormal blood clotting.
The result should be interpreted together with the patient's clinical history and other thrombophilia tests.
Important note about the results
Functional Protein C testing can be influenced by anticoagulant medications, vitamin K status, liver function, acute thrombosis, and other clinical conditions.
An abnormal result should therefore be interpreted carefully and, when appropriate, confirmed with repeat testing.
When Protein C deficiency is suspected, your doctor may consider measuring Protein C activity together with Protein C antigen to help distinguish between different types of deficiency.
Functional Protein C may also be evaluated together with Free Protein S, Antithrombin, Factor V Leiden, Prothrombin G20210A, and other thrombophilia investigations when clinically indicated.
The diagnosis of Protein C deficiency should be based on appropriate laboratory testing together with the patient's personal and family history of thrombosis.
Prefer to talk to someone first? Contact us to book.
