Quantitation of 5-Methyltetrahydrofolate in Cerebrospinal Fluid Using Liquid Chromatography-Electrospray-Tandem Mass Spectrometry

We describe a simple stable isotope dilution method for accurate and precise measurement of cerebrospinal fluid (CSF) 5-methyltetrahydrofolate (5-MTHF) as a clinical diagnostic test. 5-MTHF is the main biologically active form of folate and is involved in the regulation of homocysteine and numerous methylation reactions, including synthesis of neurotransmitters, lipids, DNA, and RNA. Measurement of 5-MTHF in CSF provides diagnostic information regarding disorders affecting folate metabolism within the central nervous system, in particular inborn errors of folate metabolism and cerebral folate deficiency. Determination of 5-MTHF in CSF (50 μL) was performed utilizing high-performance liquid chromatography coupled with electrospray positive ionization tandem mass spectrometry (HPLC-ESI-MS/MS). 5-MTHF in CSF is determined by a 1:2 dilution with internal standard (5-MTHF-13C5) and injected directly onto the HPLC-ESI-MS/MS system. Each assay is quantified using a five-point standard curve (25-400 nM) and has an analytical measurement range of 3-1000 nM.