What is heparin's mechanism of action and how is its effect monitored?

• A. Enhances antithrombin III to inhibit thrombin and factor Xa; monitored by d-dimer.
• B. Enhances antithrombin III to inhibit thrombin and factor Xa; monitored by aPTT.
• C. Directly inhibits platelets; monitored by bleeding time.
• D. Inhibits vitamin K epoxide reductase; monitored by PT/INR.

Answer: (B)

Heparin works by binding to antithrombin III and greatly increasing its ability to inhibit thrombin (factor IIa) and factor Xa, which slows the final steps of the coagulation cascade and prevents clot formation. Because it acts indirectly through antithrombin III, the standard way to monitor unfractionated heparin is the activated partial thromboplastin time (aPTT). As heparin activity rises, aPTT lengthens, so clinicians adjust the dose to keep the aPTT within a therapeutic range (often a multiple of the baseline value). D-dimer is not used to monitor heparin; it indicates fibrin degradation and is more about diagnosing thrombosis. The other statements describe mechanisms or monitors for different drugs—direct platelet inhibitors or warfarin—rather than unfractionated heparin.