Background: The combined measurement of methylmalonic acid (MMA) and total homocysteine (tHcy) in serum or plasma is useful in diagnosing and distinguishing between cobalamin and folate deficiencies. We developed and validated an isotope-dilution gas chromatography-mass spectrometry (GC-MS) method with automated sample workup for the determination of MMA, tHcy, and the related amino acids Met, total cysteine (tCys), Ser, and Gly in serum or plasma.
Methods: Serum or plasma samples (100 microL) were treated with a reductant (dithioerythritol), deproteinized with ethanol, and derivatized and extracted in a single step by the addition of methylchloroformate and toluene. All liquid handling was performed in 96-well (1 mL) microtiter plates by a robotic workstation. The N(S)-methoxycarbonyl ethyl ester derivatives were analyzed by GC-MS in the selected-ion monitoring mode.
Results: Detection limits (signal-to-noise ratio, 5:1) were between 0.03 micromol/L (MMA) and 10 micromol/L (Ser, tCys). The assay was linear to 100 micromol/L for MMA and tHcy and to 1000 micromol/L for Met, tCys, Ser, and Gly. The within-day CVs ranged from 0.7% to 3.6% (n = 20), and the between-day CVs from 2.1% to 8.1% (n = 20). The recovery was between 79% and 99% for the different analytes.
Conclusion: This assay combines a simple and automated sample preparation with selective and sensitive GC-MS analysis and is well suited for the combined measurement of MMA, tHcy, and the related amino acids.