Introduction:
Various point-of-care (POC) devices are available for measure- ment of PT-INR with whole blood in patients receiving vitamin K antagonists (VKAs). However, the variability in values measured with various POC devic- es and traditional plasma-based laboratory measurement in clinical practice has yet to be clarified.

Material and methods:
PT-INR values measured with the same blood sam- ple by POC devices of INRatio/INRatio 2 (INRatio) and the CoaguChek XS portable INR monitor (CoaguChek) were compared with those measured with traditional plasma-based laboratory measurement (lab-INR) in 1,347 measurements from 393 patients. The difference between the PT-INR values obtained from Lab-INR and POC devicees were expressed as Δ. To quantify the relationships among values obtained by various methods, regression analysis and the Bland-Altman method were applied.

Results:
PT-INR values measured with INRatio were higher than lab-INR with Δ = –0.33 ±0.38 (mean ± SD; 95% CI: –0.31 – –0.35). Pearson’s correlation coefficient between the values measured with INRatio and lab-INR was 0.784. The PT-INR values measured with CoaguChek were not different from lab-INR with Δ –0.08 ±0.15 (mean ± SD; 95% CI: –0.07 – –0.09). The correla- tion between the value obtained with CoaguChek XS and lab PT-INR was high with Pearson’s correlation coefficient 0.951.

Conclusions:
Even with the blood obtained from the same patients treated with the same dose of warfarin, the PT-INR values measured with INRatio were higher than traditional laboratory measurements. The apparent “qual- ity of warfarin control” is influenced by the methods of PT-INR measure- ments.