Comprehensive Respiratory and Burn Care Practice Test

Monitoring unfractionated heparin therapy relies on the activated partial thromboplastin time because heparin slows the intrinsic and common coagulation pathways by enhancing antithrombin III. This prolongation of the aPTT reflects the level of anticoagulation, so clinicians adjust the heparin dose to keep the aPTT within a therapeutic range (often 1.5–2.5 times the control value, per the lab protocol). Prothrombin time and INR assess the extrinsic pathway and are used to monitor warfarin therapy, not heparin. Platelet count is watched to detect heparin-induced thrombocytopenia, not for dosing the drug. Thus, the lab value used to monitor heparin therapy is the activated partial thromboplastin time.