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Comprehensive and Detailed Explanation:

The activated partial thromboplastin time (aPTT) is commonly used to monitor unfractionated heparin therapy. Heparin enhances the activity of antithrombin, which inhibits several coagulation factors, particularly factors IIa and Xa. Because heparin affects the intrinsic and common pathways, aPTT provides a measure of anticoagulant effect.

The PT is primarily used to monitor warfarin therapy because it evaluates the extrinsic and common pathways. Bleeding time is an older assessment of platelet function and is not used for routine heparin monitoring. Platelet counts may be monitored during heparin therapy to evaluate for heparin-induced thrombocytopenia but do not measure anticoagulant activity.

MLS professionals performing coagulation testing must understand anticoagulant mechanisms, specimen requirements, and result interpretation. Accurate monitoring helps maintain therapeutic anticoagulation while reducing the risk of bleeding complications.