Endocrine Abstracts (2019) 65 P17 | DOI: 10.1530/endoabs.65.P17

Very high rate of false positive biochemical results when screening for phaeochromocytoma in a large, undifferentiated population with variable indications for testing

Gregory Kline1, Jessica Boyd1,2, Alexander Leung1, Andrew Tang1 & Hossein Sadrzadeh1,2

1Cumming School of Medicine, University of Calgary, Calgary, Alberta, Canada; 2Alberta Public laboratories, Calgary, Alberta, Canada

Pheochromocytoma-Paraganglioma (PPGL) is a rare but important tumour with non-specific presentations that overlap with extremely common entities such as anxiety, hypertension, acute illness and episodic ‘spells.’ Assessment of urine normetanephrine or metanephrine(UNM-M) in real life practice, where PPGL is very rare and PPGL mimics are extremely common, may show overlap in results with loss of specificity depending on the reference range chosen. We performed a retrospective review of all UNM-M performed in a central lab serving Southern Alberta over an 8 year period, encompassing 13 525 unique patients. After excluding pediatric ages and patients with CKD (eGFR< 50 ml/min), there were 12573 unique patients who had 14383 measures of UNM-M. 88 patients (0.7%) had markedly high UNM-M compatible with likely PPGL. However, depending on the age category (in decades), between 10% and 22% of all UNM results were reported to be above the upper reference limit(URL), particularly between ages of 40–60. Less than 3% had elevations in both UNM and UMN. Of those with high UNM, 96% were less than 3 times the URL. We conclude there is an extraordinarily high prevalence of high UNM seen in real life population use of the test. However, the vast majority of high UNM are unlikely to be PPGL given the disease rarity and the massive population sample size of tests performed. This suggests the current laboratory URL may be too low (high sensitivity but poor specificity) and/or that the reference range may not be appropriate to the type of patient that is being screened for PPGL. Depending on the frequency of use of any screening test in a population, if the disease of interest is rare and the specificity of the test is poor, a very high rate of false positive results will be expected.

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