The activated protein C (APC) resistance assay detects reduced anticoagulant response to APC—most often caused by the Factor V Leiden mutation. It is a functional clotting test used to evaluate unexplained venous thromboembolism, recurrent pregnancy loss, thrombosis at a young age, or a family history of thrombophilia. A positive result prompts genetic testing and helps guide anticoagulation and risk management.

High activated protein C (APC) resistance means the body’s natural anticoagulant (APC) is less able to deactivate clotting factor V, so blood clots form more easily. It’s often caused by the Factor V Leiden mutation and increases the risk of venous thromboembolism. Diagnosis uses specialized blood tests; management may include anticoagulation, controlling risk factors and tailored advice for pregnancy or surgery.

Low APCR (activated protein C resistance) means blood is less responsive to activated protein C, a natural anticoagulant. It often reflects Factor V Leiden or acquired causes and raises the risk of venous clots (deep vein thrombosis, pulmonary embolism) and pregnancy complications. Clinically it may prompt further genetic or coagulation testing and can influence decisions about anticoagulation in high‑risk situations.

The activated protein C (APC) resistance ratio is the clotting time with APC divided by clotting time without APC, often given as a normalized ratio. A value ≥2.0 is generally considered normal; a ratio <2.0 indicates APC resistance, most commonly due to Factor V Leiden, and is associated with increased venous thrombosis risk. Positive results often lead to genetic testing and risk assessment.

Treatment depends on thrombosis. For acute VTE, start anticoagulation (LMWH or heparin) then switch to an oral anticoagulant (DOAC or warfarin). Long-term or lifelong anticoagulation may be recommended for recurrent events or high risk. Use perioperative or pregnancy thromboprophylaxis (usually LMWH) and avoid estrogen-containing contraceptives. Individualize management with specialist input.

The protein C test measures the level or activity of protein C, a natural anticoagulant, to assess risk of abnormal blood clotting (thrombophilia). It’s used to investigate unexplained or recurrent venous clots, a family history of thrombosis, or before long-term anticoagulation decisions. Results can identify inherited or acquired deficiency and guide diagnosis, management and prevention of thrombotic events.