A 45-year-old man with acute pulmonary embolism receives weight-based unfractionated heparin but fails to achieve therapeutic aPTT despite escalating doses. He has a history of a prior unprovoked DVT. Which laboratory finding best explains the heparin resistance?

• A Positive lupus anticoagulant
• B Low plasma antithrombin activity ✓
• C Elevated factor VIII level
• D Anti-PF4 heparin antibodies

Explanation

Unfractionated heparin works by binding antithrombin and accelerating its inhibition of thrombin and factor Xa. Congenital or acquired antithrombin deficiency leaves little cofactor for heparin to potentiate, producing apparent heparin resistance with persistently subtherapeutic aPTT. Treatment options are high-dose heparin, antithrombin concentrate, or switching to a direct thrombin inhibitor. Anti-PF4 antibodies would suggest HIT, where the problem is thrombocytopenia and thrombosis rather than failure to prolong the aPTT.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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