An attempt was made to correct for the effects of coagulometers on the International Sensitivity Index (ISI) in a series of collaborative studies. Modified ISI were derived from the prothrombin time results with coagulometer systems using a range of calibrated plasmas. Two alternative approaches to correction of the ISI were evaluated. The first relied on the consensus orthogonal regression slopes of the prothrombin times for each coagulometer system plotted against the consensus manual results; the second depended on the local individual slope of the prothrombin times for each instrument. The two procedures were compared with the currently recommended method where International Normalised Ratios (INR) are derived from the manual ISI of the thromboplastin. The recommended method gave a significant bias from the manual results with most coagulometers. In contrast, the local correction procedure gave no significant biases, whereas the consensus method did so in a few instances. Both these correction procedures seem more reliable than the recommended method of INR derivation, but the local correction is more accurate and offers a more practical solution by allowing laboratories to determine their own corrected ISI on a range of calibrated plasmas.
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