Effective and safe treatment with oral anticoagulants is possible if the results of laboratory control (prothrombin time measurements) are expressed in terms if the International Normalized Ratio (INR).
The INR can be calculated with the formula: INR = (PT/MNPT) ISI in which PT is the patient's prothrombin time, MNPT is the mean normal PT, and ISI the international sensitivity index of the measurement system. The ISI of the system should be provided by the reagent (thromboplastin) manufacturer. ISI calibration of the system is performed with international reference preparations or certified reference materials.
The second reference material for rabbit thromboplastin (CRM149R) was used extensively and it was anticipated that the stock would be exhausted by 1993. As a consequence, CRM149R was to be replaced by a similar reference material.
The objective of the project was to establish the parameters of the orthogonal regression line describing the relationship between log (clotting time) with the current WHO international reference preparation of thromboplastin rabbit, plain (coded RBT/90) and log (clotting time) with the proposed third reference material for rabbit thromboplastin.
Parameters were estimated independently by 11 laboratories. The orthogonal regression line is described as Y=A+B.X where Y=log (PT/s) determined with RBT/90 and X=log (PT/s) determined with the proposed third reference material for rabbit thromboplastin.
The certified values of the third reference material for rabbit thromboplastin (CRM149S) are:
slope (B): 1.257 (mean value), 0.013 (standard error)
intercept (A): -0.242 (mean value), 0.019 (standard error).
The B value is equal to the ISI for CRM 149S.
The B value is used for the ISI calibration of other working thromboplastins.
Prothrombin times were determined manually on a total of 217 normal plasmas and 621 plasmas from patients under long-term oral anticoagulant therapy, by 11 laboratories using RBT/90 and the proposed third reference material. Accelerated degradation studies, homogeneity testing and stability testing after reconstitution of the lyophilized material were performed by one laboratory.