From Dr. John Olson, University of Texas Medical Center in San Antonio:
Hello George, Happy New Year! I hope that your holiday was as pleasant for you as mine was for me. I read with interest the brief piece on the reference interval for the INR. When I was at the University of Iowa a number of years ago, we reported the international normalized ratio (INR) and the prothormbin time (PT) in seconds, the idea being that the INR would be used only to monitor vitamin K antagonists (VKA). When I would go to the floors to see patients and ask the residents or fellows what the PT was, I would uniformly get the answer in INR no matter what the etiology; most patients are not on VKA. Thus, there is a need for the reference interval for the INR to be published with the result, as well as the therapeutic interval.
In my mind, the only exception would be the INR performed at the point of care in the anticoagulation clinic. In that setting, the only application would be evaluation of VKA and the reference interval may not be useful. In all other settings, patients will be evaluated for all conditions that may require a PT and the upper limits of normal are necessary. The CAP does currently require that the INR have a reference interval (this is being challenged) but I believe that the reference interval is important. The query asked if the interval can be derived from the reference interval for the PT in seconds and the answer is yes. It is, after all, just math. Thanks, John.
Thank you, Dr Olson. This is an important update to my 11.4.11 comment to Patti Richardson and Dr. Manjula Balasubramanian, St Christopher’s Hospital for Children, Philadelphia, PA in which I suggested the INR reference interval was optional.