Colorimetric method for estimating methylmalonic acid in urine.

The increased urinary excretion of methylmalonic acid in vitamin B12 deficiency has attracted attention and several studies have suggested that it has clinical value as a test of vitamin B12 deficiency (Cox and White, 1962; Bashir, Hinterberger, and Jones, 1966). Chromatographic methods of estimation have been used but are either time consuming or only semiquantitative over a narrow range of methylmalonic acid levels. Giorgio and Plaut (1965) described a quantitative colorimetric method using the coupling of methylmalonic acid with diazotised p-nitroaniline after purification of urine by ion exchange resin. The resin they used (Dowex 3) is no longer available. As the rapid detection of methylmalonic aciduria has potential use in the clinical laboratory in patients in whom multiple haematinic deficiencies may be present, it was decided to try to develop a suitable method using this colour reaction.