Methylmalonic acid (MMA) is an organic acid that accumulates in the blood and urine when vitamin B12 is deficient, as B12 is an essential cofactor for the enzyme that converts methylmalonyl-CoA to succinyl-CoA in this metabolic pathway. MMA is the most sensitive and specific biomarker for functional vitamin B12 deficiency at the cellular level.
Unlike serum B12, which can appear within the normal range while cellular B12 function is impaired (often due to high holotranscobalamin being depleted while total B12 appears adequate), elevated MMA confirms that B12 is functionally insufficient at the tissue level. MMA testing is particularly valuable in older adults, vegans, and people on metformin or PPIs, where B12 deficiency is common but serum levels can be misleading.
FAQs
Is MMA better than serum B12 for detecting deficiency?
Yes, in many cases. Serum B12 can appear normal while functional B12 deficiency exists at the cellular level. MMA is a functional marker showing whether B12-dependent enzymes are actually working adequately. It is particularly valuable when serum B12 is borderline.
What does elevated MMA with normal B12 mean?
It suggests functional B12 insufficiency despite apparently adequate serum levels. This can occur with high total B12 from inactive forms, or when tissue stores are depleted despite normal serum transport. B12 supplementation is typically warranted.
Does kidney disease affect MMA?
Yes. Impaired kidney function reduces MMA clearance, causing elevated blood MMA without true B12 deficiency. This must always be considered when interpreting MMA results in people with reduced eGFR.
How quickly does MMA normalise after B12 treatment?
MMA typically normalises within 4-8 weeks of effective B12 replacement. Monitoring MMA alongside B12 levels confirms treatment is correcting the functional deficiency.
