Methylmalonic acid (MMA) is a dicarboxylic acid that is a C-methylated derivative of malonate.
The coenzyme A linked form of methylmalonic acid, methylmalonyl-CoA, is converted into succinyl-CoA by methylmalonyl-CoA mutase, in a reaction that requires vitamin B12 as a cofactor.
Vitamin B 12 is a cofactor for methylmalonyl-CoA mutase, , an enzyme that converts methylmalonyl-CoA into succinyl CoA a part of the citric acid cycle.
Increased levels may indicate a vitamin B12 deficiency.
However, it is sensitive test but not specific.
It is elevated in 90-98% of patients with B12 deficiency.
Over the age of 70 years 20-25% of patients have elevated levels of MMA, but 25-33% of them do not have B12 deficiency.
May be elevated in patients with renal disease.
Excess levels are associated with methylmalonic acidemia.
Concentrations in blood are measured by Gas chromatographic Mass spectrometry or LC-MS and the expected values of MMA in healthy people are between 73-271 nmol/L.
Among patients with a low-normal B12 level 5 to 10% will have elevated serum methylmalonic acid concentrations indicating cellular B12 deficiency.
By adding a methylmalonic acid level to a screen of patients with length-dependent peripheral neuropathy the finding of a cellular B12 deficiency as the cause of neuropathy will occur 2 to 8% of cases.
When vitamin B 12 levels are inconsistent with clinical findings, serum methylmalonic acid assay is an alternative diagnostic test.