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Unreliability of International Normalized Ratio for Monitoring Warfarin Therapy in Patients with Lupus Anticoagulant
Author(s) -
Rosborough Terry K.,
Shepherd Michele F.
Publication year - 2004
Publication title -
pharmacotherapy: the journal of human pharmacology and drug therapy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.227
H-Index - 109
eISSN - 1875-9114
pISSN - 0277-0008
DOI - 10.1592/phco.24.9.838.36102
Subject(s) - warfarin , lupus anticoagulant , anticoagulant therapy , medicine , thrombosis , atrial fibrillation
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.