Accuracy of point of care coagulometers compared to reference laboratory measurements in patients on oral anticoagulation therapy.

Background: Vitamin K antagonists (VKA) are widely prescribed throughout the world. Patients on VKA therapy require international normalized ratio (INR) monitoring of venous blood to ensure the response remains within the therapeutic window. Point-of-care devices (POC-INR) can safely and easily monitor VKA efficacy but need to be evaluated in practice. The aim of this study was to assess the precision and accuracy of a new POC-INR (Qlab) compared to the laboratory plasma technique and the CoaguChek-XS system.

Methods: Consecutive patients on VKA referred to our institution were included. The study was designed to analyze 75 patients divided equally in the following subgroups: INR<2; INR=2-3; INR>3. INR was measured with an established laboratory method (INRREF) with an international sensitivity index of 1.0 and by two different POC-INRs: the Qlab (INRQlab) and the CoaguChek-XS systems (INRXS).

Results: 82 patients treated mainly for atrial fibrillation or venous thromboembolism disease were included. Precision in therapeutic range (INR=2-3) of both POC-INRs was satisfactory with a coefficient of variation of 4.6% for the Qlab and 4.3% for the CoaguChek-XS. INRRef was 2.70 ± 1.36, INRQlab 2.59 ± 1.25 and INRXS 2.89 ± 1.37. Accuracy was low with the Qlab (R(2)=0.64) and higher with the CoaguChek-XS (R(2)=0.94). The mean relative difference from the INRRef was higher for the Qlab (18.4%) than for the CoaguChek-XS (12.9%). Clinical concordance was lower with the Qlab (78.2%) than with the CoaguChek-XS (90.0%).

Conclusion: This study suggests that the Qlab has accuracy limitations with clinical consequences. New POC-INR devices require careful evaluation prior to clinical implementation.