Isotope dilution analysis of methylcitric acid in amniotic fluid for the prenatal diagnosis of propionic and methylmalonic acidemia.

A stable isotope dilution assay for methylcitric acid in amniotic fluid was developed to provide rapid prenatal diagnosis of the inherited disorders propionic acidemia and methylmalonic acidemia. The method utilizes two 2H3-labeled diastereoisomers of methylcitric acid as internal standards, isolation by liquid partition chromatography and quantitation of the trimethyl esters by chemical ionization selected ion monitoring gas chromatography-mass spectrometry. Methylcitric acid at a concentration of 0.38 +/- 0.10 mumol/l was detected in normal amniotic fluid. Highly elevated levels of 7.87 and 9.16 mumol were found in the fluids surrounding fetuses affected with propionic acidemia and levels of 1.79, 2.72 and 12.27 mumol were found for fetuses with methylmalonic acidemia. Methylcitric acid was not elevated in the amniotic fluid of a fetus heterozygous for propionic acidemia. In the five pregnancies at risk for propionic acidemia, and three pregnancies at risk for methylmalonic acidemia, the levels of methylcitric acid in amniotic fluid gave the diagnosis in all cases. Measurement of methylcitric acid in amniotic fluid therefore provides a rapid and reliable method for the prenatal diagnosis of these genetic disorders.