Protein C is a natural anticoagulant that inactivates coagulation factors Va and VIIIa. Resistance to activated protein C (APC) is a relatively frequent finding in patients with unexplained or familial venous thromboembolism (VTE). This abnormality is associated in over 95% of cases with a single point mutation in the factor V gene (factor V R506Q), called factor V Leiden. The mutation alters an APC cleavage site on factor V, making it resistant to inactivation by APC.
This defect is highly prevalent, occurring in the heterozygous form in 5% of healthy Caucasians, and in 30-50% of patients with recurrent VTE. It is the most common hereditary cause of thrombosis, imparting an 8-fold increased risk of VTE in heterozygotes and an 80-fold increased risk of VTE in homozygotes.
The APC resistance clotting test is an important component of laboratory panels for venous thrombosis. A normal result rules out factor V Leiden, while an abnormal result should be followed up with a PCR assay to confirm the presence of factor V Leiden. The degree of abnormality of the APC-resistance assay correlates with heterozygosity or homozygosity for the factor V Leiden mutation.
Some APC resistance assays are based on a modified activated partial thromboplastin time (APTT). One aliquot of patient plasma is diluted in factor V-deficient plasma while a second aliquot is not. The sample is incubated with a standard amount of phospholipid and a contact activator. Calcium is added and the length of time to form a clot is measured. The ratio of the test with and without added factor V-deficient plasma is reported as the APC resistance. The sensitivity for detecting the presence of a factor V Leiden mutation is 99%.
The reference value is an APC resistance ratio of 2.3 or higher. A result less than 2.3 suggests abnormal resistance to APC. The ratio for heterozygous carriers is usually 1.5 to 1.8, while the ratio for homozygous patients is 1.1 to 1.4.
The assay is unreliable in patients with abnormally elevated prothrombin time (PT) or APTT. Lupus anticoagulants interfere with these assays, which is a problem in view of the high prevalence of both APC resistance and lupus anticoagulants in thrombophilic populations. The presence of heparin, direct thrombin inhibitors, and director factor Xa inhibitors can produce discrepant results in APC resistance and factor V Leiden assays.
Specimen requirement is one 5mL blue-top tube containing 3.2% sodium citrate. Both the coagulation and PCR tests can be performed on the same sample.
References
Dahlback B. Resistance to activated protein C as risk factor for thrombosis: molecular mechanisms, laboratory investigation, and clinical management. Semin Hematol. 1997;34(3):217-234.
Grody WW, Griffin JH, Taylor AK, Korf BR, Heit JA; ACMG Factor V. Leiden Working Group. American College of Medical Genetics consensus statement on factor V Leiden mutation testing. Genet Med. 2001;3(2):139-148
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