Determination of the Mean Normal Prothrombin Time for Assessment of International Normalized Ratios

Abstract

SummaryTo report a Prothrombin Time (PT) as International Normalized Ratio in controlling oral anticoagulant therapy, the Mean Normal PT (MNPT) is required. To correct for methodological differences in performing the PT test, each laboratory should determine its own MNPT for each batch of reagent using fresh blood samples from a large number of normal individuals. This would be a laborious procedure. Two models for simplified assessment of MNPT were investigated by two laboratories in a collaborative study. According to the models, the MNPT of a new batch of reagent is calculated, using the PT of a lyophilized control plasma measured with the new batch and a reference batch, as well as the MNPT of the reference batch obtained with fresh samples. Experimental results were obtained with 19 batches of bovine thromboplastin, 4 lyophilized normal control plasmas and fresh blood samples of 40 normal individuals. The PTs of the 4 lyophilized normal control plasmas were not identical to the MNPT of the fresh normal samples and also different from each other. Therefore, the uncorrected PTs of these control plasmas cannot be used as MNPT.In general, there was good agreement between measured and calculated MNPT, although some control plasmas gave better results than others. There were no significant differences between the results obtained by both calculation models.

Home prothrombin time monitoring: A literature analysis

“…To correct for methodological differences in performing the PT test, each laboratory should determine its own MNPT for each batch of reagent using fresh blood samples from a large number of individuals [45]. Usually, home monitoring devices are provided with MNPTs encoded in the test cartridge or in a batch specific code chip [40].…”

Precision And Accuracy Of The International Normalized Ratio In Oral Anticoagulant Control

“…Precision of the mean normal PT For calculation of the INR, the mean normal PT (MNPT) is required. It is strongly recom mended that MNPT is determined locally on fresh samples of normal individuals (9,10). The criteria for defining normal plasma will not be considered here.…”

Long-Term French Experience in INR Standardization by a Procedure Using Plasma Calibrants

“…However, the use of INR continues to be problematic. [6][7][8][9] Various reports highlight difficulties encountered in the following: (1) determination of an accurate International Sensitivity Index (ISI) related to a problem of calibration precision because of the reference thromboplastin calibration 10 and to the manufacturer's calibration of the commercial thromboplastin 1 1 -1 2 ; (2) the choice of the normal plasma value used in the PT ratio 13,14 ; and (3) above all, the instrument-dependency of the ISI, [15][16][17][18][19] because the ISI is still determined manually according to World Health Organization r e c o m m e n d a t i o n s , 1 whereas most laboratories now use coagulometers. The ISI given by the manufacturer is therefore no longer valid for i n s t r u m e n t -m e a s u r e d PTs.…”