INR comparison between the CoaguChek Pro PT(N) and a standard laboratory method

The prothrombin time (PT), also called thromboplastin time ("Quick"), is usually measured using citrated plasma from venous blood. Recently, portable coagulation monitors have been developed, which measure PT using non-anticoagulant capillary whole blood from a finger stick. In the present study, we compared the International Normalized Ratio (INR) of the standard laboratory method (INRven) with the newly developed reagent PT(N) for the CoaguChek Pro portable monitor (INRcap) in various patient groups: healthy individuals, patients with oral anticoagulation and patients with a deficiency of coagulation factor V, factor VII or factor X, respectively.

One hundred and fifty-five patients were included in this prospective open comparison study. Capillary PT was measured with the portable coagulation monitor CoaguChek Pro using the new disposable cartridge PT(N) (containing rabbit brain thromboplastin). In comparison, PT was measured using citrated venous plasma and the reagent Innovin on the coagulation analyzer STA-R.

We found a correlation coefficient of 0.85 between capillary and venous INR values among the 100 patients with oral anticoagulation. The slope of the regression line was 1.4 and the y-intercept is -0.65. Agreement between both methods was found to be 80% (95% CI: 72-88%) and the standard-agreement was 85% (95% CI: 78-92%). Among the 30 healthy subjects, the individual differences between INRven and INRcap were in 4 cases 0, in 21 cases 0.1 and in 5 cases 0.2.

The new test cartridge PT(N) was found to be a valuable tool for measuring PT among healthy subjects. However, among patients with oral anticoagulation, agreement between INRcap measured with the new cartridge PT(N) and INRven was only moderate. Our results show that improvements are necessary for a more valuable measurement of capillary PT with portable coagulation monitors.