Activated Protein C Resistance (Factor V Leiden) Assay

The normal reference value for the activated protein C resistance (APCR) ratio is greater than 2.1.

See the list below:

• Heterozygous carriers of factor V Leiden typically have APCR ratios in the range of 1.5 -1.8.

• Homozygotes for factor V Leiden usually have APRC ratios of less than 1.5.

Specifics for collection and panels are as follows:

• Specimen type: Platelet-poor plasma

• Container: Vacutainer, light blue top (citrate; see the image below)

  Light blue top (citrate).

• Collection method: Venipuncture

• Specimen volume: 3 mL

Other instructions are as follows:

• Spin down, remove plasma, and spin plasma again

• Freeze 1-mL aliquots of the specimen at 40°C or less within 4 hours

Panel

Venous thrombosis panel

Related tests include the following:

• Antithrombin

• Protein C

• Protein S

• Homocysteine

• Methylenetetrahydrofolate reductase (MTHFR)

• Factor V Leiden mutation PCR assay

Description

Activated protein C (APC) is the enzymatically active form of protein C after proteolytic cleavage by thrombomodulin-bound thrombin. An important natural anticoagulant, APC inactivates factors Va and VIIa. APC resistance (APCR) is a hypercoagulability disorder in which factor V cannot be inactivated by APC. In the vast majority of cases, the patient with APCR carries the Leiden variant of factor V, in which the APC cleavage site on factor V is altered. The factor V Leiden mutation is not uncommon, with the heterozygous carrier state occurring in up to 5% of the US population. Factor V Leiden is found in 30-50% of patients with recurrent venous thromboembolism (VTE) and is the most common hereditary cause of thrombosis.[1, 2, 3, 4]

APCR is characterized by a reduced anticoagulant response after a standard amount of APC is added to a plasma specimen. The APCR assay uses snake venoms to activate factor V and prothrombin, thus triggering the lower portion of the clotting pathway and eliminating interference by factor VIII and by lupus anticoagulants (in the absence of phospholipid complexes). Although the APCR assay is no longer based directly on the activated partial thromboplastin time (aPTT), it is expressed as a ratio of aPTT with APC:aPTT without APC. The assay is highly sensitive and specific for factor V Leiden, with excellent discrimination between normal individuals and factor V Leiden heterozygous carriers. The assay is not affected by anticoagulation with standard or low molecular weight heparin or with warfarin.

Indications/Applications

Indications for APCR ratio testing include the following:

• Incident or recurrent VTE

• Family history of VTE

• Recurrent miscarriage or complications of pregnancy

• History of arterial thrombosis in a younger patient (< 50 yrs)

• History of arterial thrombosis in a patients with no other risk factors for atherosclerosis

• Patients with abnormal APCR ratios should have genetic testing to confirm factor V Leiden mutation status.

A study by Morimont et al indicated that the interlaboratory reproducibility and intralaboratory precision are excellent for the endogenous thrombin potential (ETP)–based APCR assay, an assay recommended for use in contraceptive agent development. The investigators found evidence that the normalized APC sensitivity ratio (nAPCsr; which measures resistance to APC and is computed using the ETP), when derived through a validated and standardized methodology, is appropriately sensitive no matter which laboratory is employing it. The report states “that the nAPCsr could become a promising regulatory and clinical tool to identify the thrombogenicity of” combined oral contraceptives.[5]