Low-molecular-weight heparin may alter point-of-care assay for international normalized ratio.
OBJECTIVE
To compare the international normalized ratio (INR) measured by a point-of-care testing device with that measured by a reference laboratory method for patients receiving either warfarin only or warfarin plus low-molecular-weight heparin (LMWH).


METHODS
Retrospective study.


METHODS
Outpatient anticoagulation clinic.


METHODS
Ninety-one patients receiving warfarin for various indications; 59 of them receiving only warfarin and 32 receiving warfarin plus LMWH.


METHODS
Capillary blood was obtained for INR determination by a point-of-care device, and venous blood was obtained for INR determination in a standard reference laboratory.


RESULTS
Ninety-one patients had INR pairs run on a point-of-care device and by the laboratory. In both the patients receiving only warfarin and in those receiving warfarin plus LMWH, the mean INR as determined by the point-of-care testing device was statistically significantly higher than the mean INR determined by the laboratory. Although the differences were statistically significant in both groups, the clinical significance of this difference was accentuated in the patients receiving warfarin plus LMWH. The measure of divergence between the point-of-care and laboratory methods was greater in the group receiving warfarin plus LMWH than in the warfarin-only group, with a mean +/- SD percent change between the INR values of 24.19 +/- 27.54% in the warfarin plus LMWH group and 7.21 +/- 17.73% in the warfarin-only group. In assessing the clinical impact of such variability, a greater degree of discordance in dosing adjustment decisions was noted for patients receiving warfarin plus LMWH. In this group, a 25% rate of discordance was noted compared with 8% in the warfarin-only group. Such discrepancy in dosing decisions based on the point-of-care INR would have resulted in discontinuation of LMWH therapy before the patient acquired a true therapeutic INR, with use of the laboratory measurement.


CONCLUSIONS
The INR measured with the point-of-care device in patients receiving concurrent LMWH and warfarin therapy may be inaccurate. Patients receiving LMWH in addition to warfarin should have INRs checked by means of the standard reference laboratory method.
Phillips
Elizabeth M
EM
Department of Medicine, State University of New York Upstate Medical University, Syracuse, New York, USA.
Buchan
Debra A
DA
Newman
Nancy
N
Rajan
Arun
A
Zia
Shams
S
D000328:Adult; D000368:Aged; D000925:Anticoagulants / Q000494:pharmacology*; D005260:Female; D006495:Heparin, Low-Molecular-Weight / Q000494:pharmacology*; D006801:Humans; D019934:International Normalized Ratio; D008297:Male; D008875:Middle Aged; D019095:Point-of-Care Systems; D012189:Retrospective Studies
D000925:Anticoagulants; D006495:Heparin, Low-Molecular-Weight