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Isopropanol protein precipitation for the analysis of plasma free metanephrines by liquid chromatography-tandem mass spectrometry.

Marney LC, Laha TJ, Baird GS, Rainey PM, Hoofnagle AN

Department of Laboratory Medicine, University of Washington, Seattle, WA, USA.

BACKGROUND: High-performance liquid chromatography-tandem mass spectrometric (LC-MS/MS)1 analysis of plasma free metanephrines is the most diagnostically sensitive and specific screening test for the diagnosis of pheochromocytoma. We sought to develop an in-house method for this expensive test METHODS: We used off-line isopropanol protein precipitation of plasma to remove interfering substances before LC-MS/MS analysis. We compared the extraction efficiency and limits of quantification of protein precipitation to those of previously reported solid-phase techniques. RESULTS: The new method had limits of quantification of 0.09 nmol/L and 0.17 nmol/L for metanephrine and normetanephrine, respectively. Method comparison with a previously described solid-phase extraction method revealed Deming regression slopes of 0.904 and 0.994, intercepts of 0.007 and 0.023, and SEs of the residuals (S(y/x)) of 0.071 and 0.284 for metanephrine and normetanephrine, respectively. Extraction efficiency of isopropanol protein precipitation was 66% for metanephrine and 35% for normetanephrine, results that were superior to the efficiencies of 4% and 1% for our adapted solid-phase extraction method. No ion suppression was observed at the retention times for metanephrine and normetanephrine. CONCLUSIONS: Isopropanol protein precipitation is a novel and effective off-line sample preparation method for metanephrines that offers a less expensive alternative to on-line solid-phase extraction for low-volume testing and requires a sample volume of only 200 microL. The mass spectrometric analysis time is equivalent to that of solid-phase techniques.

Published 30 September 2008 in Clin Chem, 54(10): 1729-32.
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