Which test first suggests Factor V Leiden?

• A. Activated protein C resistance assay
• B. Prothrombin time only
• C. Partial thromboplastin time only
• D. D-dimer assay

Answer: (A)

Factor V Leiden changes how the coagulation system responds to activated protein C, so the best way to raise suspicion for it is a test that directly measures APC resistance. The activated protein C resistance assay does just that: it assesses how much the clotting time is prolonged when APC is added. In individuals with Factor V Leiden, Factor V remains active longer because it’s resistant to inactivation by APC, so the clotting time isn’t prolonged as much as expected. This makes APC resistance the first test that points toward the possibility of Factor V Leiden, prompting confirmatory genetic testing for the mutation. PT and aPTT by themselves don’t specifically probe the APC pathway, and a D-dimer reflects active thrombosis rather than inherited risk, so they’re not the first clues for this condition.