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Coagulation laboratory tests

International normalized ratio (INR)

Reviewer: Jeremy Parsons, M.D. (see Reviewers page)
Revised: 10 February 2013, last major update November 2012
Copyright: (c) 2002-2013, PathologyOutlines.com, Inc.


● Used to standardize prothrombin time (PT) results for patients taking warfarin (coumadin)
● Intended to make comparisons similar between different labs by compensating for variable thromboplastins used in PT test
● Often used as an arbitrary level of “bleeding likelihood” (J Clin Pathol 2003;56:48 (recommendations for reporting))
● Generally only helpful when a patient is on warfarin and one is evaluating if patient is currently in warfarin therapeutic ranges, or if a warfarin reversal is being considered for upcoming intervention
● Patient self management may be effective (J Clin Pathol 2002;55:845 (patient self testing))


● Therapeutic goal is usually a value of 2 to 3
● Defined as patient PT divided by mean normal PT, with the result raised to the power (exponent) of the ISI
● Results can be improved with a calibration curve (Arch Pathol Lab Med 2004;128:308)
● Results are affected by different thromboplastin reagents, not by storage at room temperature for up to 24 hours (Arch Pathol Lab Med 1998;122:972)

End of Coagulation > Coagulation laboratory tests > International normalized ratio (INR)

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