The Reliability of Point-of-Care (POC) Prothrombin Time Testing - A Comparison of CoaguChek XS® INR Measurements with Hospital Laboratory Monitoring.

Background The development of Point of Care (POC) testing devices enables patients to test their own International Normalised Ratio (INR) at home, and has lead to the development of patient self- testing (PST) programmes. This facilitates more frequent testing of anticoagulation levels which in turn improves time in therapeutic range (TTR) and reduces bleeding and thrombotic complications in patients on oral anticoagulation therapy (OAT).¹ External quality control of POC devices is essential to ensure that results are robust. One approach is to collect a venous sample at the same time as POC testing, which is analysed using an appropriate hospital laboratory analyzer. It has been recommended that INR results should be within 0.5 of each other when measured by each method². However, previous studies have shown that when compared with clinical laboratory values, POC devices may show statistically significant differences between the INR values.³ Aim The aim of this study is primarily to compare TTR and adverse events between two different approaches to OAT management and more specifically in this presentation to examine the intra patient variability of INR measurements performed by two different techniques.

Methods 160 patients routinely attending the anticoagulant clinic of Cork University Hospital (CUH) were enrolled to take part in a prospective randomized controlled, cross-over design trial of supervised PST of warfarin therapy using an internet based expert system (Coagcare®, ZyCare, Inc) and POC testing device (CoaguChek XS®, Roche, UK) vs. rountine medical care in CUH. During the 6 months PST arm of the study a venous blood sample was taken from each patient for laboratory determination of the INR on 3 occasions; at enrolment, 2 months and 4 months. This measurement was then compared with the INR measured on the Coaguchek XS® machine using capillary blood from the patient’s fingertip.

Results 88 patients have being enrolled in the PST arm of the study to date and 454 POC measurements have been compared with laboratory values. Good correlation has been shown between the two methods of INR determination (r² = 0.8314), however, statistically significant differences did occur (p < 0.005). A Bland and Altman plot showed good agreement between the dual measurements at values between 2.0–3.5 INR units. However there seemed to be increasing disagreement between the two methods as the INR rose above 3.5. Four patients were excluded from the study because the differences in the INR measurements were greater than 0.5 INR units. Conclusion POC testing devices for OAT are reliable and safe. External quality assessment should be inherent to any programme involving these devices. Clinicians involved in the use of POC anticoagulation monitoring should be aware that POC and laboratory values may differ and intervene if there is not adequate correlation between the two INR measurements.