Introduction: Anticoagulation therapy with warfarin is monitored by the prothrombin time (PT) assay. The PT is standardized using international normalized ratios (INRs). By keeping the INR within specific values, it is possible to reduce potential complications from the treatment. To facilitate the PT monitoring, point-of-care devices suitable for capillary whole blood measurements have been developed. The aims of this study were to compare the INR values obtained by such a device, CoaguChek S, with those obtained from the Owren-type PT assay and to evaluate the differences seen.
Materials and methods: In 351 consecutive warfarin-treated patients, INR was measured in capillary whole blood samples with CoaguChek S and was compared to venous plasma samples analyzed with the Owren PT method. Sixty-nine of these patients, including those deviating the most between the methods, were further evaluated according to levels of factor II (prothrombin), factor V, factor VII, factor X, fibrinogen, activated partial thromboplastin time (aPTT) and antiphospholipid antibodies.
Results: The results from CoaguChek S produced a correlation coefficient of 0.81 to the Owren-type PT assay and a concordance of 85.2%. Factor V and fibrinogen correlated significantly (p<0.05) to the degree of deviation between the methods. The presence of antiphospholipid antibodies did not influence the degree of deviation between the two methods.
Conclusions: INR analysis of whole blood with CoaguChek S is comparable with INR measured in plasma with Owren chemistry. The activities of factor V and fibrinogen contribute to the deviation seen between the methods. Differences in sensitivity to antiphospholipid antibodies could not be demonstrated.