1013-112 Is the INR an Adequate Corrective Factor for the Range of Thromboplastins Utilized in North America?

Abstract

For a given intensity of anticoagulant therapy the Prothrombin Time Ratio (PTR) shows wide variability when measured with different thromboplastin reagents due to differing sensitivities of the commercially available thromboplastins as reflected in differences in the International Sensitivity Index (ISI) values for each reagent. The International Normalized Ratio (INR) is used to correct for this variation. We investigated the variability of INR obtained with 11 different thromboplastins from 4 different manufacturers using a single venipuncture sample from each of 49 unselected patients. The ISI values of the thromboplastins ranged from 1.0 to 2.81 and all analyses were performed on a MLA 700 instrument. For any individual thromboplastin the coefficient of variation (CV) was very low–1.6 to 4.6%. However, when comparing INR values from all 11 thromboplastins the CV was 19%, which was a significant improvement over the CV for PTR of 29%, (p < 0.005) but still high enough to allow for clinically significant differences as demonstrated by the following confidence ranges: INR 95% Confidence Range CV lower limit upper limit 1.6 1.2 2.0 13% 2.4 1.7 3.1 15% 3.7 2.0 5.3 23% 4.7 2.2 7.3 27% The CV and the standard deviation were directly related to the INR with higher variability at higher levels of anticoagulant intensity. While the INR offers a significant reduction in variance as compared to the PTR, it is suboptimal for the range of thromboplastins available in North America, and can result in clinically significant variation when comparing INR results between institutions utilizing different thromboplastins, especially at higher intensities of anticoagulation.