Platform
Company
Immunology & Autoimmune
Review status
Currently under review
Pending specialist review and validation.
This test looks at how your blood responds to activated protein C, a natural substance that slows clotting. In a laboratory clotting assay, activated protein C is added to your plasma and the time it takes to form a clot is measured. If your clotting does not slow as expected, it suggests resistance to activated protein C.
APC resistance is most often caused by a genetic change in the factor V gene, commonly called factor V Leiden. It can also be acquired in certain situations, such as during pregnancy or with some medications. The test is usually part of an evaluation for a tendency to form blood clots.
People with APC resistance have a higher chance of developing clots in the veins, such as deep vein thrombosis or pulmonary embolism. Your clinician may order this test if you have had an unexplained clot, a strong family history of clots, or certain pregnancy complications, or before starting estrogen therapy when risk is a concern.
Results can help guide next steps, such as whether to perform genetic testing for factor V Leiden, consider preventive strategies during high-risk periods, or adjust plans for surgery, pregnancy, or hormonal treatments. It is one piece of a broader thrombophilia workup and is interpreted alongside your history and other tests.
A result showing a normal response to activated protein C suggests that the natural brake on clotting is working as expected. A reduced response suggests APC resistance and may prompt follow-up testing, often including a DNA test for factor V Leiden. Your clinician will consider your medications, recent health events, and other coagulation tests to confirm the meaning of the result.
Certain medicines and conditions can alter this assay, so an unexpected result may need repeat testing when you are well and, if safe, when you are not taking interfering drugs. A normal result does not rule out all causes of clotting, and an abnormal result does not mean you will definitely develop a clot. Discuss what your result means for your personal risk and whether any preventive steps are recommended.
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.
Heparin, direct oral anticoagulants, and warfarin can change clotting assay behavior and lead to misleading results. Never stop or adjust these medicines without your clinician’s guidance; timing of the test may be adjusted.
Lupus anticoagulant and related antibodies can interfere with clot-based assays and mimic resistance or blunt the expected response. Additional tests may be needed to sort out the cause.
Pregnancy, combined hormonal contraception, or hormone therapy can produce an acquired form of APC resistance. Your clinician may interpret results with this in mind or repeat testing at a different time.
A recent clot, infection, surgery, or high factor levels can affect results. Testing is often most informative when you are clinically stable and away from acute events.
Underfilled citrate tubes, delayed processing, or improper storage can alter clotting times. Proper collection and prompt handling help ensure accurate results.
Very high or low hematocrit and liver disorders can change coagulation factor balance and influence the assay, requiring careful interpretation.
References