A simplified and rapid quantitative assay for propionic and methylmalonic acids in urine

Propionic and methylmalonic aciduria occur individually in inborn errors of metabolism and together in vitamin B₁₂-deficient states. A method is described for the simultaneous and rapid extraction of these acids from urine and their quantification by a simple gas chromatographic technique. The assay is based upon the spontaneous and quantitative decarboxylation of methylmalonic acid (MMA) at 225 °C. to its monocarboxylic acid product, propionic acid. By utilizing another substituted malonic acid, ethylmalonic acid, as a specific internal standard, accurate quantitation is possible by peak height ratio analysis. Endogenous propionic acid is then measured at 130 °C., a temperature at which methylmalonic acid does not decarboxylate. The assay is rapid with a total running time of approximately 2 hours. The method provides excellent resolution of propionic acid excretion at or above 0.5 mg. per liter of urine. The level of resolution for methylmalonic acid, in the presence of propionic aciduria, was 5 mg. per liter of urine.