The purpose of this study is to see if the CoaguChek XS is accurate in measuring International Normalized Ratio (INR) in patients with Antiphospholipid Antibody Syndrome (APL) receiving warfarin therapy.

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.

Inclusion Criteria:

• 18-65 years of age
• anticoagulated with warfarin for at least 1 month
• Diagnosis of antiphospholipid antibody syndrome for intervention arm

Exclusion Criteria:

• any type of mental disability that would hinder their ability to give informed consent
• any terminal illness or any other condition that could interfere with study completion, that person will be excluded.

Patients presenting to UF/Shands Hematology Anticoagulation Clinic for a routine office visit with pharmacy.