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...

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Bibliographic Details
Published in:Pharmacotherapy 2004-07, Vol.24 (7), p.838-842
Main Authors: Rosborough, Terry K., Shepherd, Michele F.
Format: Article
Language:English
Subjects:
Adult
Aged
Aged, 80 and over
chromogenic factor X activity
Female
Humans
INR
international normalized ratio
International Normalized Ratio - standards
lupus anticoagulant
Lupus Coagulation Inhibitor - blood
Male
Middle Aged
Prospective Studies
Reproducibility of Results
Statistics, Nonparametric
Warfarin - therapeutic use
warfarin therapy
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Summary: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.
ISSN:0277-0008
1875-9114
DOI:10.1592/phco.24.9.838.36102