Accuracy and clinical usefulness of the CoaguChek S and XS Point of Care devices when starting warfarin in a hospital outreach setting

ObjectiveTo assess clinical usefulness of CoaguChek S and XS monitors to measure International Normalised Ratio (INR) when starting warfarin in community patients.MethodsINR in consecutive patients starting warfarin plus enoxaparin was measured in the laboratory and on capillary blood at home using CoaguChek S or XS point of care (POC) devices. INR was measured daily until > 2.0 for 2 consecutive readings. Linear regression and Bland Altman plots were derived to compare POC with laboratory INR. Percentages of POC measurements within 0.5 and 0.8 units of laboratory INR <2.0, 2.0-3.5 and > 3.5 were calculated. Clinical utility was assessed using previously reported criteria.Results200 CoaguChek S and 337 CoaguChek XS results were obtained from 57 and 98 patients respectively and paired with laboratory values. Correlation was acceptable between CoaguChek S and laboratory INRs (r2 = 0.7732), and excellent between CoaguChek XS and laboratory INRs (r2 = 0.9514). Bland-Altman plots showed an increasing difference between laboratory INR above 3.0 for CoaguChek S INRs but no systematic bias with increasing CoaguChek XS INRs. 83.5% of CoaguChek S and 93.5% of CoaguChek XS INRs were within 0.5 units of laboratory INR. 90% of CoaguChek S and 99.4% of the CoaguChek XS INRs were within 0.8 units of laboratory INR. Clinical agreement occurred in 89% and 99.7% of cases by expanded criteria and 82% and 99.4% of cases by narrow criteria when using CoaguChek S and CoaguChek XS respectively.ConclusionsThe CoaguChek XS is suitable for outpatient INR monitoring when starting warfarin.