Comparison of International Normalized Ratios provided by two point-of-care devices and laboratory-based venipuncture in a pharmacist-managed anticoagulation clinic

Abstract

The International Normalized Ratios (INRs) measured by two point-of-care (POC) devices were compared with those obtained via laboratory-based venipuncture. In an outpatient, pharmacist-managed anticoagulation clinic, 52 patients receiving oral warfarin therapy had their INR values measured using two POC devices, i-STAT PT/INR (Abbott Laboratories, Abbott Park, IL) and CoaguChek XS Plus (Roche Diagnostics, Indianapolis, IN). At the same visit, one venous blood sample was collected from each patient for INR measurement by the reference laboratory's instrumentation (STAGO, Diagnostica-Stago, Parsippany, NJ). Accuracy was evaluated by calculating the absolute difference for each set of INR values. Clinical correlation was defined as an INR measurement obtained by the POC devices that would have resulted in the same therapeutic decision as the INR value measured by the reference laboratory's instrumentation. Accuracy was superior with the CoaguChek XS Plus device. The absolute difference (mean ± S.D.) in the INR measurements obtained using STAGO versus CoaguChek XS Plus was 0.28 ± 0.31 (p < 0.0001). The absolute difference in INR values measured using STAGO and the i-STAT PT/INR device was 0.51 ± 0.44 (p < 0.0001). For clinical correlation, 17 (33%) of 52 INR measurements with the CoaguChek XS Plus were sufficiently different from the STAGO-measured INR values to have resulted in a different therapeutic decision (p < 0.001), compared with 28 (54%) of 52 with the i-STAT PT/INR (p < 0.001). INR measurements generated by POC devices exhibited positive bias, compared with laboratory-based venipuncture, for INR values at the high end of the INR range.