Rapid analysis of metanephrine and normetanephrine in urine by gas chromatography-mass spectrometry.

Background: Widely used HPLC methods for quantification of metanephrine and normetanephrine in urine often have long analysis times and are frequently plagued by drug interferences. We describe a gas chromatography-mass spectrometry method designed to overcome these limitations.

Methods: Metanephrine and normetanephrine conjugates were converted to unconjugated metanephrine and normetanephrine by acid hydrolysis. To avoid the rapid decomposition of the deuterated internal standards (metanephrine-d(3) and normetanephrine-d(3)) under hydrolysis conditions, the internal standards were added after hydrolysis. Solid-phase extraction was used to isolate the hydrolyzed metanephrines from urine. Samples were concentrated by evaporation, then derivatized simultaneously with N-methyl-N-(trimethylsilyl)trifluoroacetamide and N-methyl-bis-heptafluoro-butryamide at room temperature.

Results: The assay was linear from 25 to 7000 microg/L. The intraassay CVs were < 5 % and the interassay CVs < 12%. Comparison with a routine HPLC method (n = 192) by Deming regression yielded a slope of 1.00 +/- 0.02 microg/L, an intercept of -5.8 +/- 7.8 micro/L, and S(y/x) = 50.6 microg/L for metanephrine and a slope of 0.94 +/- 0.03, intercept of 19 +/- 11 microg/L, and S(y/x) = 60 microg/L for normetanephrine. The correlation coefficients (r) were calculated after log transformation of the data and gave r = 0.97 for metanephrine and r = 0.97 for normetanephrine. Interference from common medications or drug metabolites was seen in <1% of samples. The time between sequential injections was < 7 min.

Conclusions: This new gas chromatography-mass spectrometry assay for total fractionated metanephrines is rapid, compares well with a standard HPLC assay, and avoids most drug interferences that commonly affect HPLC assays for urine metanephrines.