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INR Reference Range?

A message from Patti Richardson and Dr. Manjula Balasubramanian, St Christopher’s Hospital for Children, Philadelphia, PA (originally posted November, 2011):

Is it necessary to have a reference range for the international normalized ratio (INR)? It is a calculated value. If so, should these be derived from the normal prothrombin time (PT) ranges? The Heme CAP checklist does not address this question but the Gen CAP requirement is for all reported values to have reference ranges. Thanks for your help.

Hello, Ms Richardson and Dr Balasubramanian, and thank you for your question. Laboratory directors typically compute and publish an INR reference interval (reference range, normal range) from the PT reference interval in addition to publishing the target therapeutic range. In theory, all you should need is the PT reference interval in seconds, and I doubt a CAP assessor would make an issue if a lab did not publish an INR reference interval. However, the INR reference interval is helpful to those of us who, thinking that INR is the name of the assay, don’t automatically refer to the PT result in seconds. Geo.

Added May 2, 2013: This note relates to the March 14, 2013 post “No More PT in Seconds” from Dr. Larry Brace.

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