Differences in warfarin dosing decisions based on international normalized ratio measurements with two point-of-care testing devices and a reference laboratory measurement

Study Objectives. To assess the accuracy of warfarin dosing decisions and the degree of numeric bias between two point-of-care devices using a local reference laboratory's international normalized ratio (INR) as the standard measure, and to determine the relationship between dosing decisions and INR values obtained with the point-of-care devices. Design. Prospective study. Setting. Outpatient anticoagulation clinic. Subjects. Two hundred two patients taking oral warfarin and 10 control subjects. Interventions. For the two point-of-care devices, AvoSure and ProTime, the finger-stick method was used to collect capillary blood samples in each subject. At the same visit, one venous blood sample was collected from each subject for the laboratory analysis. Measurements and Main Results. Dosing agreement was assessed as the proportion of agreement between each device and the laboratory in terms of maintenance dosage adjustments (increase, decrease, or no change). The level of agreement between each device and the laboratory was evaluated by dosing agreement analysis, bias analysis, and concordance coefficient analysis. In the dosing agreement analysis, 78% of INR values from the AvoSure device would have resulted in the same dosing decision as that with the laboratory INR values compared with 66% from the ProTime device (p<0.001). The mean bias for the ProTime device (0.5 ± 0.4 INR units) was significantly higher (p=0.005) than that for the AvoSure device (0.4 ± 0.5 INR units). The ProTime device overestimated low INR values to a greater extent than did the AvoSure device. Concordance between the laboratory measurement and each device was similar (ρ_c = 0.82 for ProTime, ρ_c = 0.76 for AvoSure). Conclusions. Assessing dosing decisions yielded distinct, useful clinical information. The AvoSure device is associated with less systematic bias and a higher degree of clinical agreement with our reference laboratory measurement than those of the ProTime device.