INR Accuracy Affected by LMWH Therapy

Anna D. Garrett, PharmD, BCPS, CPP
Published Online: Wednesday, February 1, 2006

Patients who need to receive concomitant warfarin and low-molecular-weight heparin (LMWH) may have inaccurate international normalized ratio (INR) measurements on point-of-care (POC) testing devices. A recent study that included 91 outpatients compared INR readings on POC devices using capillary blood with venous blood analyzed on a standard reference laboratory analyzer. The investigators found that the mean INR on the POC devices was significantly higher statistically than the mean value determined by the laboratory in patients who were on both warfarin and LMWH. These discrepancies would have resulted in different dosing decisions for 25% of the double therapy group versus 8% of the warfarin-only group. If POC readings alone had been used to make therapeutic decisions, LWMH would have been discontinued before the patient had a true therapeutic INR as measured by the laboratory, thereby putting the patient at risk for clotting events.

LMWH and warfarin may be used together as outpatient therapy for deep vein thrombosis or as "bridge"therapy for patients who must discontinue warfarin for surgery or other procedures. The results of this small study suggest that laboratory confirmation of the INR may be prudent before stopping LMWH therapy.

Dr. Garrett is a clinical pharmacist practitioner at Cornerstone Health Care in High Point, NC.

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