Title: Accuracy and clinical utility of the CoaguChek XS portable international normalised ratio monitor in a pilot study of warfarin home-monitoring Running title: Evaluation of the CoaguChek XS

Aim: To evaluate the accuracy of the CoaguChek XS international normalised ratio (INR) monitor compared to the laboratory method. Methods: The accuracy and ease of use of the recently marketed CoaguChek XS portable INR monitor was evaluated in 17 patients involved in a trial of warfarin home-monitoring. INR results from the monitor were compared to the laboratory method. Clinical applicability was measured by discrepant INR values, defined in the literature by expanded and narrow agreement criteria, and by the proportion of INR values differing by more than 15% and 20% from the laboratory method. Results: Participants provided 59 comparison INR measurements for analysis. The paired results were highly correlated (r = 0.91). Expanded and narrow agreement between paired INR values occurred 100% of the time. Only three CoaguChek XS (5.1%) results differed by >15% compared to the laboratory method; no results differed by more than 20% or were more than 0.5 INR units discrepant. Conclusions: In the hands of patients the CoaguChek XS demonstrated good correlation with laboratory determination of INR and compared very well with expanded and narrow clinical agreement criteria. Both patients and doctors were highly satisfied with the accuracy and ease of use of the CoaguChek XS.