Falsely elevated INRs in warfarin-treated patients with the lupus anticoagulant.

The Lupus Anticoagulant (L.A.) is an antibody that prolongs the clotting time of in-vitro laboratory tests by binding phospholipid in the test system. Patients with the L.A. are at increased risk for development of venous and arterial thrombosis but not hemorrhage. Therefore, many patients with the L.A. are being treated with warfarin sodium to prevent reoccurrence of thrombosis. This oral anticoagulant therapy is traditionally regulated by periodic determination of the Prothrombin Time (PT). This test is usually unaffected by the L.A. However, we have recently identified a small series of patients with the L.A. in whom the PT is affected by the L.A. This interference is manifest as an artifactually increased International Normalized Ratio (INR). These patients were identified by failure to achieve significant correction of the PT with addition of an equal volume of normal plasma to the patient plasma and a Factor X level discordant with the PT INR Interference in determination of the PT by the L.A. was found to occur in 6.5% of patients identified with the L.A. by our laboratory. It is suggested that patients with this complication of anticoagulant therapy be monitored by measurement of Factor X levels rather than the PT INR. Failure to recognize this complication may result in inadequate anticoagulation and recurrent thrombosis.