Serum methylmalonic acid by capillary zone electrophoresis using electrokinetic injection and indirect photometric detection.

Elevated methylmalonic acid (MMA) is a superior marker for cobalamin (vitamin B12) deficiency. A rapid assay for serum MMA suitable for clinical specimens has been developed. The specimen is injected electrokinetically after a simple extraction. MMA is measured using indirect photometric detection at 210 nm, based on the displacement of phthalic acid in the running electrolyte; no derivatization is needed. The detection limit is 0.2 mumol/L, well below 0.4 mumol/L, which defines elevated concentrations. The assay is linear up to 100 mumol/L, although specimens above 10 mumol/L are diluted to maximize resolution. The CV is < 10% at 0.30 mumol/L and 0.80 mumol/L. The assay gives results that are comparable to more traditional methods.