Test not affected by heparin?

• A. Thrombin time
• B. Prothrombin time
• C. Reptilase time
• D. Activated partial thromboplastin time

Answer: (C)

The concept to focus on is how heparin affects clotting tests. Heparin inhibits thrombin and related steps in the coagulation cascade, so tests that depend on thrombin activity or thrombin generation (like the prothrombin time, the activated partial thromboplastin time, and the thrombin time) are prolonged in the presence of heparin. Reptilase time, however, uses reptilase (a venom enzyme) to convert fibrinogen to fibrin and is not inhibited by heparin, so this test remains normal despite heparin in the sample. That resistance to heparin makes reptilase time the correct choice for a test not affected by heparin.