A 25-year-old male presents to his physician complaining of leg pain. The physician diagnoses a deep vein thrombosis (DVT) and wants to determine the cause of the thrombotic episode. Which laboratory test may be used to investigate the hypercoagulable states?

• A. PT
• B. aPTT
• C. APCR
• D. Euglobulin lysis

Answer: (C)

A key concept here is inherited hypercoagulability, most commonly due to resistance to activated protein C caused by Factor V Leiden. Activated protein C normally inactivates Factor Va to dampen thrombin generation. If Factor V Leiden makes Factor Va resistant to APC, the anticoagulant effect of APC is blunted, so the clotting tendency remains higher.

The test for this resistance compares clotting times with and without activated protein C. In someone with APC resistance, the presence of APC does not markedly prolong the clotting time, resulting in a low APC resistance ratio. This functional screen specifically points to a hypercoagulable state from APC resistance, commonly due to Factor V Leiden.

PT and aPTT are general tests of the coagulation pathways and don’t specifically detect APC resistance. Euglobulin lysis time assesses fibrinolysis rather than predisposition to thrombosis. Therefore, the Activated Protein C Resistance test best addresses the question of an inherited hypercoagulable state in a young patient with DVT.