That is an excellent question, for which I do not have an answer. When I asked my rheumatologist how could I have PMR with a normal ESR, he said that although my ESR was within the normal range now, it could have been much lower in the weeks or months prior to the onset of symptoms. In other words, normal ESR for one person may be at or below the lower end of the statistically defined normal range. The onset of an inflammatory disease state could therefore cause an elevation into the mid or upper level of the normal range. In short, PMR must be managed primarily by symptom level, and not test results.