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

Abstract

Objective: To assess clinical usefulness of CoaguChek S and XS monitors to measure International Normalised Ratio (INR) when starting warfarin in community patients.

Methods: INR 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.

Results: 200 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 (r(2)=0.7732), and excellent between CoaguChek XS and laboratory INRs (r(2)=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.

Conclusions: The CoaguChek XS is suitable for outpatient INR monitoring when starting warfarin.