Plasma-Free Metanephrine and Normetanephrine Quantification for Clinical Applications Validated by Combining Solid-Phase Extraction and HPLC-MS/MS.

Plasma-free metanephrines are the most sensitive and specific biochemical markers for diagnosing catecholamine-secreting tumors, such as pheochromocytoma and paraganglioma. In this study, we developed and validated a liquid chromatography-tandem mass spectrometry method for quantifying metanephrine and normetanephrine in human plasma, using solid-phase extraction with a weak cation-exchange mechanism. Validation was performed according to the FDA Bioanalytical Method Validation Guidance and CLSI guideline C62-A. The method showed excellent linearity over concentration ranges of 0.11-13.92 nmol/L for metanephrine and 0.14-26.43 nmol/L for normetanephrine, with correlation coefficients exceeding 0.999. The accuracy, precision, and lower limit of quantification met the acceptance criteria of the study. Matrix effect evaluation revealed a process efficiency of 121% for metanephrine at the lowest concentration, slightly exceeding the acceptable range of 100 ± 15%. This was likely because of matrix-induced ion enhancement or variability in extraction efficiency. However, all other tested concentrations were within the acceptable limits. Overall, this method demonstrated high sensitivity, specificity, and reproducibility, making it suitable for routine clinical applications. Minor deviations at low concentrations do not compromise reliability; however, future optimizations, such as matrix-matched calibration, may further improve performance.