We previously highlighted the problem of frequent false positives in 24 hour urine normetanephrine(UNM) measurements owing to reference intervals that are inappropriately low for the population being screened for pheochromocytoma. Using a large population database, we devised new age-stratified reference intervals for the 24 hour UNM test that were higher compared to previous. However, it was uncertain as to whether this would compromise test sensitivity for true pheochromocytoma cases.
Retrospective analysis of all pheochromocytoma cases from a recently constructed provincial registry. All confirmed cases had their diagnostic UNM results retrospectively re-analysed according to the newly proposed UNM reference intervals to determine the percentage and phenotype of cases that might have been theoretically missed with the new reference range.
After excluding pediatric and non-secretory head and neck paragangliomas, there were 60 confirmed pheochromocytoma cases. Using prior reference intervals, 51/60 (85%) had an abnormally high UNM. Of the 9 with normal UNM, 4 had a high urine metanephrine(UMN), 5 had normal levels of both UNM and UMN such that 55/60 had abnormal test results, representing the historical combined test sensitivity of 92%. Using the proposed reference interval, 43/60 (72%) had high UNM results. Of the 17 with normal UNM, 12 had high UMN, 5 had normal levels of both UNM and UMN. Therefore, 55/60 patients had had elevations in either UNM or UMN, corresponding to an identical combined test sensitivity of 92%.
Reference intervals for UNM derived from actual clinical population screening data are higher than in traditional healthy volunteers. Use of these more appropriate reference intervals can significantly reduce the false positive rate without compromising test sensitivity for true pheochromocytoma.

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