Antiphospholipid Antibody Syndrome (APL) is an autoimmune disease that increases ones risk for blood clots. Therefore, these patients receive anticoagulation therapy with warfarin. Point of care devices such as the CoaguChek XS are often used to monitor International Normalized Ratio (INR) in patients on warfarin. However, the antibodies present in patients with APL may lead to false INR results when using the CoaguChek XS. This study will compare the accuracy of the CoaguChek XS in measuring INR in patients with APL by measuring INR by the CoaguChek XS and a standard lab as a reference point. Patients on warfarin for indications other than APL will be measured by both methods as well to serve as a control group.