Transiently Increased variation between a point-of-care and laboratory inr method after a long period of correlation: A case study demonstrating the need for ongoing correlation of poc with the central laboratory

Objectives: To perform long-term comparison between laboratory Stago and Point-of-Care (POC) i-STAT methods for determining the international normalized ratio (INR). Methods: This was a multicenter method comparison of patient INR results and factors related to performance variance. Results: 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. Conclusions: This study emphasizes the need for continual laboratory correlation with POC devices and caution in using published comparisons.