Patients with a lupus anticoagulant (LAC) may present with thrombosis, and abnormal coagulation studies. It is important to recognize that these individuals may require special laboratory tests other than measuring the activated partial thromboplastin time or prothrombin time/international normalized ratio (PT/INR) traditionally used by clinicians to monitor hep-arin and warfarin. We recommend the use of a factor Xa inhibition test to regulate heparin in this patient population. This test measures the ability of heparin, as a cofactor of antithrombin III, to inMbit the catalytic function of factor Xa in plasma. Therapeutic levels are 0.3-0.7 U/mL of anti-Xa activity. For warfarin, we suggest measuring the factor X levels as determined by a chromogenic assay, if the patient initially has a prolonged PT/ INR. Therapeutic levels are 10%-40% of normal values. In this article, we review this and other tests available to monitor antithrornbotic therapy in patients with the LAC.