Background: The standardized test used for evaluating the effect of warfarin is the prothrombin time (PT) which is measured and expressed in international normalized ratio (INR). Regular control of treatment intensity is required since inappropriate dosage increases the risk for complications. Portable point-of-care analytical instruments for measurement of capillary whole blood PT have been available for the last decades. The purpose of this study was to compare and evaluate INR values obtained by the point-of-care device CoaguChek XS, to Owren PT in a hospital setting.
Materials and methods: In 397 warfarin-treated patients, capillary whole blood was analyzed with the CoaguChek XS and the results were compared to analysis of venous plasma samples with the Owren PT assay. To study reproducibility and rule out preanalytical errors, a subgroup of 152 patients had two capillary blood samples analyzed with the CoaguChek XS.
Results: In 397 patients, with a median age(+/-2SD) of 69.0(50-88) years, there was a positive correlation between results from the CoaguChek XS and the Owren-type PT assay (r=0.94;p<0.001) and concordance of 88.2%. The mean INR difference (S.D) was 0.02 (0.22). Comparison of the 152 double samples analyzed with the CoaguChek XS, produced a positive correlation of 0.99; p<0.001.
Conclusions: The CoaguChek XS presents reproducible, highly comparable results with Owren PT at therapeutic levels of INR. The CoaguChek XS seems to produce better results than the earlier CoaguChek S, probably due to a new method of PT measurement where levels of fibrinogen and haematocrit do not affect the outcome.