Methylmalonic acid concentrations in serum of normal subjects: biological variability and effect of oral L-isoleucine loads before and after intramuscular administration of cobalamin.

The clinical value of measuring concentrations of methylmalonic acid in serum (S-MMA) as an aid in the diagnosis of cobalamin deficiency has recently aroused interest. In 58 healthy subjects, ages 40-68 years, we found a 0.95 reference interval of 0.05-0.37 mumol/L (mean 0.21, SD 0.094). In 33 of the subjects, who were studied further, day-to-day variation (SD) was 0.031 mumol/L. Intake of food had no effect. Weekly and three-monthly intra-individual variations were both 0.038 mumol/L. In all seven subjects with S-MMA greater than 0.30 mumol/L, the concentrations declined significantly after intramuscular administration of cobalamin. No significant difference was found between mean serum cobalamin concentrations in these seven and in the remaining subjects. We have also established the normal response of S-MMA to standardized oral loading of L-isoleucine: 100 mmol caused a significant average S-MMA increase of 0.072 mumol/L before cobalamin administration vs 0.013 mumol/L after cobalamin, without significant relation to initial S-MMA values. Our results provide a necessary background for interpretation of S-MMA measurements in clinical studies.