Unfractionated heparin therapy is conventionally monitored by activated partial thromboplastin time (aPTT), targeting 1.5 to 2.5 times the control value. In which situation is the aPTT unreliable and an anti-Xa assay preferred?

• A Presence of lupus anticoagulant or very high heparin doses ✓
• B Concurrent aspirin therapy
• C Recent clopidogrel loading
• D Mild thrombocytopenia unrelated to heparin

Explanation

Lupus anticoagulant prolongs phospholipid-dependent clotting tests including aPTT independently of heparin, and at high heparin concentrations the aPTT plateaus, so both situations give misleading values. The chromogenic anti-Xa assay measures heparin activity directly and is unaffected. Antiplatelet drugs such as aspirin and clopidogrel act on platelets and do not distort the aPTT-based heparin monitoring.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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