UFH monitoring is performed with which laboratory test?

• A. PT/INR
• B. aPTT
• C. D-dimer
• D. Platelet count

Answer: (B)

The main idea is that unfractionated heparin is best monitored by a test that reflects the activity of the intrinsic coagulation pathway. The activated partial thromboplastin time measures how long it takes plasma to clot after activation of the intrinsic pathway; heparin prolongs this time by enhancing antithrombin’s inhibition of thrombin and factor Xa. Because of this, aPTT is used to gauge the anticoagulant effect of UFH and guide dosing (with target ranges typically set by institutional protocols).

In contrast, PT/INR assesses the extrinsic pathway and vitamin K–dependent factors (and is used for warfarin therapy), D-dimer indicates fibrin degradation and helps evaluate thrombosis, and platelet count monitors for thrombocytopenia or HIT. Some settings may also use anti-Xa levels to monitor heparin, but aPTT remains the standard test for UFH monitoring.