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Thromb Haemost 1988; 60(01): 018-020
DOI: 10.1055/s-0038-1647626
Original Article
Schattauer GmbH Stuttgart

A Simple Nomogram for the Derivation of International Normalised Ratios for the Standardisation of Prothrombin Times

L Poller
The UK Reference Laboratory for Anticoagulant Reagents & Control (WHO Collaborating Centre), Withington Hospital, Manchester, UK
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Further Information

Publication History

Received 18 December 1987

Accepted after revision 11 March 1988

Publication Date:
30 June 2018 (online)

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Summary

The WHO international scale of reporting prothrombin time results is based on the calibration of local and commercial thromboplastin reagents used in the test against an international reference preparation to derive an International Sensitivity Index (ISI) for each batch of local reagent. This quantifies an individual reagent’s responsiveness to the coumarin induced defect.

Once the ISI has been assigned, the derivation of International Normalised Ratios (INR) for reporting results depends on the use of a scientific calculator by which the local ratio is raised to the power of the ISI of the local reagent or use of an alternative formula and logarithms.

A system for interpretation of INR, without the need for calculations, is presented for the known range of thromboplastins between 1.0 and 3.0 ISI in nomogram form. This chart may facilitate the general application of the INR system.

Key words

Prothrombin time - Standardisation - International Normalised Ratios - International Sensitivity Index
 

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