Effect of coagulation factors on discrepancies in International Normalized Ratio results between instruments

Background: The reasons for discrepancies between International Normalized Ratio (INR) results determined by point-of-care-instruments and laboratory measurements are not fully understood. In this study we investigated whether different levels of coagulation factors in the plasma of patients can explain some of the systematic and/or random parts of the difference in INR between the instruments. Methods: Blood samples were collected at four different patient visits from each of 34 outpatients on warfarin treatment. INR was determined on a laboratory instrument (STA Compact (R)) and on three point-of-care instruments (Simple Simon (R) PT, CoaguChek (R) XS and INRatio (TM)). In addition, the level of fibrinogen, coagulation factors II, V, VII and X was determined. INR instruments were compared in pairs. Simple linear regressions as well as multiple linear regressions and nested ANOVA analyses were used to examine the data. Results: The coagulation factors, especially fibrinogen, factors II and VII, could explain between 16% and 45% of the total variance of the differences in INR between instruments dependent on instruments compared. After correction for factors no systematic difference was seen for four of the six comparisons and the between- and within-subject variation of the differences were reduced by up to 69% and 52%, respectively. Conclusions: By correcting for the appropriate coagulation factors, especially the systematic differences, but also the between-and within-subject variation of the differences between instruments, were reduced. This indicates that different levels of coagulation factors in the plasma of the patients play an important role in explaining discrepancies between INR instruments.