No effect of acidification or freezing on urinary metanephrine levels

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Urinary metanephrine is a reliable method to estimate catecholamine secretion. Traditionally, urinary metanephrines are collected into chilled containers containing hydrochloric acid (HCl) and most laboratories freeze urinary samples before analysis. It is uncertain if these pre-analytic procedures alter metanephrine values.


To evaluate if acidifying and freezing urine samples affect the accuracy of urinary metanephrine measurements.


Random urine samples from healthy individuals were collected. Urine samples were distributed into two containers: with HCl 50% homogenized with urine to obtain pH < 2, and without HCl. Each container was divided again into aliquots for immediate measurement or freezing. One aliquot with acid (group 1) and another without acid (group 2) were sent immediately to the laboratory for testing (HPLC), while the other two aliquots, one with acid (group 3) and another without it (group 4) were frozen for 3 months at − 20 °C. Bland–Altman’s test was used to analyze inter-assay agreement between measurements.


A total of 15 individuals were included (mean age 27.5 ± 5.9 years, 8 male and 14 white). No difference was observed on mean urinary metanephrine/creatinine ratio between groups: group 1: 0.23 ± 0.11, group 2: 0.22 ± 0.07, group 3: 0.25 ± 0.13, group 4: 0.25 ± 0.15 mg/g creatinine; P > 0.05 for all the comparisons). Bland–Altman’s analysis showed agreement between the standard method (group 1) and the experimental method (group 4).


Measurement of urinary metanephrines by HPLC method is not influenced by sample acidification nor freezing at − 20 °C for 3 months.

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Fig. 1



Catecholamine O-methyltransferase


Monoamine oxidade


Aldehyde oxidase


Vanillylmandelic acid


Hydrochloric acid


Blood pressure

JNC 7:

Joint National Committee 7


High performance/pressure liquid chromatography





TMN/Cr ratio:

Total metanephrine/creatinine ratio


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Author information

LCP collected the data, performed statistical analysis, wrote the first draft of the manuscript, and revised the final version; CKK performed statistical analysis and revised the final version of the manuscript; JLC conceived the study idea, performed statistical analysis, and revised the final version of manuscript, LHC performed statistical analysis and revised the final version of the manuscript, JLG revised the final version of manuscript, CBL conceived the study idea, performed statistical analysis, and revised the final version of the manuscript. LCP is the guarantor for the contents of the article, had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Correspondence to L. C. Pinto.

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All procedures performed in the study were in accordance with the ethical standards of the institutional research committee and with the 1964 Helsinki declaration.

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Pinto, L.C., Kramer, C.K., Camargo, J.L. et al. No effect of acidification or freezing on urinary metanephrine levels. J Endocrinol Invest 43, 53–56 (2020).

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  • Urinary metanephrines
  • Acidification
  • Freezing