Determination of serum physiological concentration of methylmalonic acid by gas chromatography-mass spectrometry with selected ion monitoring.

We developed a sensitive assay for the rapid determination of serum methylmalonic acid concentration using capillary gas chromatography-mass spectrometry (GC/MS) with selected ion monitoring and a simple solid-phase extraction. The assay was linear up to 10,000 nmol/L and had a detection limit < 50 nmol/L, average recovery of 98% and between-day coefficient of variation at concentrations of 570 and 2206 nmol/L of 7.7% and 5.4%, respectively (n = 25). Comparison with another validated GC/MS method using sera with a wide range of methylmalonic acid concentrations (94-2020 nmol/L) revealed a slope and intercept of 0.97 and 17 nmol/L, respectively (n = 38). Methylmalonic acid concentrations determined by this assay in a group of apparently healthy individuals ranged from 64-331 nmol/L (n = 81). We conclude that the method is ideally suited for the determination of methylmalonic acid at physiological concentrations in both clinical and research laboratories.