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There is substantial evidence that plasma concentrations of the free (unconjugated) metanephrines metanephrine (MN) and normetanephrine (NMN) are better than other indices of catecholamine excess for detecting pheochromocytomas (1)(2)(3). However, it currently is unknown how stable these compounds are after blood collection and after separation of plasma as well as during storage. To investigate this, we modified the original method by Lenders et al. (4), which consists of HPLC with electrochemical detection, preceded by a prepurification step on cation-exchange columns, to increase the procedural recovery and thereby sensitivity. The principal adjustments were as follows:Before plasma was passed through the cation-exchange column, 1 mL of Aqua Dest and 135 μL of a solution of 0.2 mol/L ammonium acetate (pH 6.0) were added to 1 mL of plasma. The 135 μL of ammonium acetate solution included 100 μL of internal standard solution [108.7 nmol/L 3-ethoxy-4-hydroxyphenylethanolamine oxalate (EHPEA)] and 35 μL of ammonium acetate containing, only for addition experiments, MN and NMN. The calibrator mixture consisted of 19.5 nmol/L MN, 18.1 nmol/L NMN, and 108.7 nmol/L EHPEA in the aforementioned ammonium acetate solution, of which 140 μL was injected directly. After column elution, dried residues were dissolved in 150 μL of the ammonium acetate solution, of which 140 μL was injected. In all plasma samples assayed, MN and NMN peaks were completely separated from surrounding peaks by virtue of a slight increase in polarity of the mobile phase.Within-assay SDs, estimated from duplicate measurements of various samples (n = 11) containing 98–351 pmol/L MN and 129–350 pmol/L NMN, were 12.3 pmol/L for MN and 11.6 pmol/L for NMN (mean CVs, 7.0% and 4.5%, respectively). Single measurements of a control sample in 15 separate runs gave mean (SD) values of 207 (22.9) pmol/L (CV, 11%) for MN and 277 …

Stability of Plasma Free Metanephrines during Collection and Storage as Assessed by an Optimized HPLC Method with Electrochemical Detection