Accuracy of Capillary Whole Blood International Normalized Ratio on the CoaguChek S, CoaguChek XS, and i-STAT 1 Point-of-Care Analyzers

Brad S. Karon, MD, PhD; Robert D. McBane II, MD; Rajeev Chaudhry, MBBS, MPH; Lisa K. Beyer; Paula J. Santrach, MD


Am J Clin Pathol. 2008;130(1):88-92. 

In This Article

Abstract and Introduction

We evaluated the accuracy of capillary whole blood international normalized ratio (INR) on the CoaguChek S (Roche Diagnostics, Indianapolis, IN), CoaguChek XS (Roche Diagnostics), and i-STAT 1 (i-STAT, East Windsor, NJ) point-of-care (POC) analyzers compared with venous plasma INRs determined by a reference laboratory method. Overall agreement between POC and laboratory plasma INR was very good, with median bias between capillary whole blood and laboratory plasma INRs varying from 0.0 to –0.2 INR units on all devices. More than 90% of results on the CoaguChek XS and i-STAT 1 and 88% of CoaguChek S results were within 0.4 INR units of the reference laboratory method. The CoaguChek XS and i-STAT 1 demonstrated greater accuracy than the CoaguChek S as measured by the number of results that differed by more than 0.5 INR units from the reference method. Median bias between CoaguChek S capillary whole blood and laboratory plasma INRs changed over time, demonstrating the need for ongoing quality assurance measures for POC INR programs.

Point-of-care (POC) measurement of the international normalized ratio (INR) is increasingly common in outpatient, inpatient, nursing home, and home care environments. Multiple devices exist for measurement of INR from a capillary finger-stick sample, making real-time measurement and adjustment of warfarin dosages possible at sites where testing is undertaken. However, the accuracy, reliability, and safety of INR measurement by capillary finger stick remains controversial. Studies of POC INR accuracy are divided into those comparing INR measurement by capillary finger stick with matched venous plasma samples tested on a laboratory reference analyzer and those that use carefully prepared reference plasma or whole blood materials to test POC INR devices.[1,2,3,4,5,6,7]

Some previous studies comparing capillary finger stick with laboratory plasma INR values have found significant differences between results of POC and laboratory plasma INR. In these studies, the mean bias varied from –0.2 to 0.8 INR units, and percentage results within 0.5 INR units of the reference method varied from approximately 50% to approximately80%.[1,2,3] Systematic bias between POC and laboratory plasma INRs existed for all POC methods, especially at INR values of more than 3.0.[1,2,3] In contrast, 2 other studies found nearly perfect agreement between POC and laboratory plasma INRs (mean bias less than 0.1 INR unit), with 85% to 100% of values matching the reference method within 0.5 INR unit.[4,5] These various studies are difficult to compare owing to different POC devices used, differing laboratory reference methods used, and the patient populations (ie, range of INR values measured) studied.

Other investigators have prepared external quality assessment material to assess consistency between results of different POC INR devices or consistency of results from the same device used at different centers. These studies have found that the relationship between POC and laboratory plasma INRs differs by device, strip lot used for a given device, and between centers using the same device.[6,7] Another study used whole blood drawn by venipuncture to compare POC INR values on 2 devices to plasma INR results using a manual method that used World Health Organization thromboplastin standards. This study also found significant variation between POC and laboratory plasma INRs, especially at INR values greater than 3.0.[8] These studies raise significant concerns about the quality and consistency of POC INR testing and its use in outpatient practices, although it is not clear how results obtained with whole blood or external quality assessment material relate to differences between capillary whole blood and venous laboratory plasma INRs. Because the practice of self-monitoring of INR is associated with reductions in thromboembolism and major bleeding episodes,[9] it is important to identify the monitors that will provide optimal results for outpatient and self-monitoring of the INR.

In the present study, we evaluated the performance of the i-STAT 1 (i-STAT, East Windsor, NJ), CoaguChek XS (Roche Diagnostics, Indianapolis, IN), and CoaguChek S (Roche Diagnostics) POC INR monitors using capillary whole blood. The venous plasma INR from a single laboratorycoagulation analyzer with a single lot of thromboplastin was used as the reference method for comparison of device accuracy. The results provide information on the degree of systematic bias and variability between POC and laboratory plasma INRs for recently available POC INR monitors.


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