American Board of Internal Medicine (ABIM) Certification Practice Exam

Which assay is used to detect the presence of factor V Leiden (FVL)?

1. PTT assay

2. An APC resistance assay

3. Antithrombin III measurement

4. Thrombin time test

The correct answer is: An APC resistance assay

The assay that is utilized to detect the presence of factor V Leiden (FVL) is an APC resistance assay. Factor V Leiden is a genetic mutation that makes the factor V protein resistant to activated protein C (APC), which plays a crucial role in regulating blood coagulation. When a person has this mutation, their blood coagulation process is altered, leading to an increased risk of thrombosis due to prolonged clotting. The APC resistance assay specifically evaluates the functionality of factor V in response to activated protein C. In individuals without the mutation, activated protein C will cleave factor V, leading to a reduction in clotting. However, in those with factor V Leiden, this cleavage is impaired, resulting in continued clotting despite the presence of APC. Therefore, the APC resistance assay is tailored to measure this specific resistance and is highly effective in diagnosing the presence of factor V Leiden. Other assays mentioned, such as the PTT assay, antithrombin III measurement, and thrombin time test, do not provide specific information on factor V Leiden or its resistance to activated protein C. The PTT assay assesses the intrinsic and common pathways of coagulation but is not specific for factor V Leiden. Antithrombin III measurement evaluates the level of