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Influence of various confounding variables and storage conditions on metanephrine and normetanephrine levels in plasma

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Summary Objective 

Measurements of plasma free metanephrines have been advocated as first-line tests for phaeochromocytoma. The aim of the study was to assess the impact of potential confounding variables. Design 

Comparative study between 2008 and 2009. Subjects 

Hundred and eighty healthy subjects. Measurements 

The effects of age, BMI, gender, menstrual cycle (sampling every 2 days), time of day (sampling every 2 h), venepunture (0, 15, 30, 60, 90 and 120 min), physical exercise (0, 15 and 30 min), coffee (0 and 60 min), breakfast (0 and 60 min) and various body positions (standing and supine rest, each 0 and 120 min) were evaluated. In addition, whole blood and plasma samples were stored at 4 °C or at 22 °C for 0, 1, 3, 24 and 72 h. Plasma free metanephrines were measured using radioimmunoassay (LDN). Results 

While metanephrine was significantly influenced by sex and age, BMI and sex were significant predictors of normetanephrine. Coffee (+20%) and food (+8%) elevated normetanephrine significantly (P < 0·05), while metanephrine remained stable. Physical exercise increased metanephrine (+82%) as well as normetanephrine (+84%) significantly (P < 0·005). Supine rest significantly decreased both metanephrine (−34%) and normetanephrine (−19%) when compared to standing rest (P < 0·01). Metanephrine and normetanephrine were not significantly influenced by time of day, menstrual cycle or venepuncture. When plasma samples were stored at 4 °C, metanephrine and normetanephrine were stable for 72 h. Conclusions 

Physical exercise may lead to relevant changes in metanephrine and normetanephrine and should therefore be avoided prior to sampling. Although effects of age, sex and BMI were small, these variables should be considered when interpreting biochemical results. Blood should be taken in the supine position, and samples should be immediately centrifuged and stored at 4 °C to improve stability.
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Document Type: Research Article

Publication date: August 1, 2010

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