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Open Access Research article

Measurement of fractionated plasma metanephrines for exclusion of pheochromocytoma: Can specificity be improved by adjustment for age?

Anna M Sawka12*, Lehana Thabane34, Amiram Gafni4, Mitchell Levine134 and William F Young5

Author Affiliations

1 Department of Internal Medicine, St. Joseph's Healthcare, Hamilton, Ontario, L8N 4A6, Canada

2 Division of Endocrinology and Metabolism, McMaster University, Hamilton, Ontario, L8N 3Z5, Canada

3 Centre for Evaluation of Medicines, St. Joseph's Healthcare, Hamilton, Ontario, L8N 1G6, Canada

4 Department of Clinical Epidemiology and Biostatistics, McMaster University, Hamilton, Ontario, L8N 3Z5, Canada

5 Division of Endocrinology, Metabolism, Nutrition, and Internal Medicine, Mayo Clinic, Rochester, MN, 55905, USA

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BMC Endocrine Disorders 2005, 5:1  doi:10.1186/1472-6823-5-1

Published: 28 February 2005

Abstract

Background

Biochemical testing for pheochromocytoma by measurement of fractionated plasma metanephrines is limited by false positive rates of up to 18% in people without known genetic predisposition to the disease. The plasma normetanephrine fraction is responsible for most false positives and plasma normetanephrine increases with age. The objective of this study was to determine if we could improve the specificity of fractionated plasma measurements, by statistically adjusting for age.

Methods

An age-adjusted metanephrine score was derived using logistic regression from 343 subjects (including 33 people with pheochromocytoma) who underwent fractionated plasma metanephrine measurements as part of investigations for suspected pheochromocytoma at Mayo Clinic Rochester (derivation set). The performance of the age-adjusted score was validated in a dataset of 158 subjects (including patients 23 with pheochromocytoma) that underwent measurements of fractionated plasma metanephrines at Mayo Clinic the following year (validation dataset). None of the participants in the validation dataset had known genetic predisposition to pheochromocytoma.

Results

The sensitivity of the age-adjusted metanephrine score was the same as that of traditional interpretation of fractionated plasma metanephrine measurements, yielding a sensitivity of 100% (23/23, 95% confidence interval [CI] 85.7%, 100%). However, the false positive rate with traditional interpretation of fractionated plasma metanephrine measurements was 16.3% (22/135, 95% CI, 11.0%, 23.4%) and that of the age-adjusted score was significantly lower at 3.0% (4/135, 95% CI, 1.2%, 7.4%) (p < 0.001 using McNemar's test).

Conclusion

An adjustment for age in the interpretation of results of fractionated plasma metanephrines may significantly decrease false positives when using this test to exclude sporadic pheochromocytoma. Such improvements in false positive rate may result in savings of expenditures related to confirmatory imaging.