Unreliability of international normalized ratio for monitoring warfarin therapy in patients with lupus anticoagulant

STUDY OBJECTIVE: To compare the international normalized ratios (INRs) of patients positive for lupus anticoagulant and the INRs of control patients receiving warfarin therapy with equivalent therapeutic chromogenic factor X levels. DESIGN: Prospective case series. SETTING: A 625-bed, adult, private, tertiary care teaching hospital. PATIENTS: Sixty-eight outpatients positive for lupus anticoagulant and 57 control patients receiving long-term warfarin therapy. MEASUREMENTS AND MAIN RESULTS: Concomitant INR and chromogenic factor X activity were measured in all patients. In 44 control patients (77%) and 46 patients with lupus anticoagulant (68%), chromogenic factor X activity was 22-40% of normal, which is therapeutic. Of the 44 control patients, 4 (9%) had an INR above 3.0, and none had an INR above 4.0. In contrast, 18 (39%) of the 46 patients with lupus anticoagulant had an INR above 3.0, and 5 (11%) had an INR above 4.0. CONCLUSION: At least 10% of patients with lupus anticoagulant receiving long-term warfarin therapy may have falsely high INR values, which could lead to inappropriate warfarin dosage reduction. Monitoring warfarin therapy by chromogenic factor X activity in patients with lupus anticoagulant avoids this INR artifact.