Coagulation laboratory tests
International normalized ratio (INR)

Author: Jeremy Parsons, M.D. (see Authors page)

Revised: 16 May 2016, last major update November 2012

Copyright: (c) 2002-2016,, Inc.

PubMed Search: International normalized ratio [title]

Cite this page: International normalized ratio (INR). website. Accessed October 18th, 2018.
Definition / general
  • 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)
  • 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)
  • Therapeutic goal is usually a value of 2 - 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)