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. 2021 Mar 7;9(3):264. doi: 10.3390/biomedicines9030264

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© 2021 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Example of a calibration line for determination of international sensitivity index (ISI). The prothrombin time (PT) clotting times of all samples including 20 normal (red) and 60 warfarin (purple) samples are measured by the employed instrument/reagent with the reference thromboplastin and the manual technique (WHO method), and the logarithms of the PT values employed are plotted against the logarithms of the PT values assigned by using WHO method. The regression equation is calculated, and the slope of the line is established. The ISI value of the employed instrument/reagent is calculated as: ISI = slope × ISI (ref) where ISI (ref) is the ISI of the reference thromboplastin.