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A 33-year-old woman with lupus nephritis and antiphospholipid antibodies develops pulmonary embolism. The baseline activated partial thromboplastin time is 87 seconds, which is considered to be too high to be useful in adjusting subsequent anticoagulation.
Which one of the following anticoagulants would be most appropriate to adjust according to plasma anti-Xa activity in this patient?
A Heparin
B Argatroban
C Warfarin
D Lepirudin
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Answer and Critique (Correct Answer = A)
Heparin, low-molecular-weight heparins, and fondaparinux all catalyze antithrombin to neutralize clotting enzymes. Although their relative activities against specific enzymes such as thrombin differ widely from one another, all of them catalyze the inactivation of factor Xa. The presence of any of these drugs within the plasma may be assayed by incubation of the patient's plasma with activated factor Xa and a substrate that changes color when acted upon by factor Xa. The potency with which Xa is neutralized by the patient's plasma (the œanti-Xa level ) corresponds to the level of active anticoagulant drug in the plasma.
Argatroban and lepirudin are direct inhibitors of thrombin and have no effect on the clotting enzyme factor Xa. Warfarin has no direct effect on preformed clotting enzymes of any type, and does not inactivate factor Xa.
Heparin, low-molecular-weight heparins, and fondaparinux all catalyze antithrombin to neutralize clotting enzymes, and these drugs can be assayed by incubation of the patient's plasma with activated factor Xa.
