Transiently Increased Variation Between a Point-of-Care and Laboratory INR Method After a Long Period of Correlation

Abstract

To perform long-term comparison between laboratory Stago and Point-of-Care (POC) i-STAT methods for determining the international normalized ratio (INR). This was a multicenter method comparison of patient INR results and factors related to performance variance. For 5 years, the assays demonstrated close patient correlation within and above the 3.5 INR therapeutic range cutoff (bias, 0.23 INR units). Patient results above 3.5 INR were bimodal, with 60% demonstrating an i-STAT INR bias of less than 0.5. Several patient conditions were associated with the presence of a higher i-STAT bias. In year 6, a broader range i-STAT bias developed, increasing to 0.73 INR units. The increased bias persisted for 3 years, then returned to initial levels following i-STAT adjustments. The substantial increase in i-STAT bias after a long period of stability was partly corrected by renewed correlation to the international reference preparation. Additional assay drift is discussed in relation to thromboplastin reagents and other testing variables. This study emphasizes the need for continual laboratory correlation with POC devices and caution in using published comparisons.