Methylmalonic Acid (MMA) Unit Converter
Methylmalonic Acid (MMA) Unit Converter
Convert methylmalonic acid between µmol/L and nmol/L — the single most useful confirmatory test for functional B12 deficiency.
Methylmalonic Acid (MMA) converter
Mass ⇄ molarMethylmalonic acid 0.32 µmol/L
Formula and conversion factor
µmol/L = nmol/L ÷ 1000
- × 1000
- the two units differ only in scale — no molecular weight is needed
- cofactor
- vitamin B12 is the cofactor for methylmalonyl-CoA mutase; without it, methylmalonyl-CoA accumulates and is hydrolysed to methylmalonic acid
- specificity
- unlike homocysteine, methylmalonic acid does not rise in isolated folate deficiency
Worked example
Methylmalonic acid 0.32 µmol/L
0.32 × 1000 = 320 nmol/L
Reference range and interpretation
| µmol/L | nmol/L | |
|---|---|---|
| Adult reference interval | 0.05 – 0.37 | 50 – 370 |
| Confirms B12 deficiency (with low-normal B12) | > 0.37 | > 370 |
| Isolated folate deficiency | Normal | Normal |
What methylmalonic acid measures
Methylmalonic acid is the single most useful test for functional vitamin B12 deficiency. B12 is the cofactor for methylmalonyl-CoA mutase, the enzyme that converts methylmalonyl-CoA to succinyl-CoA in the final step of odd-chain fatty acid and branched amino acid catabolism. When B12 is lacking, methylmalonyl-CoA accumulates and is hydrolysed to methylmalonic acid, which rises measurably.
That mechanism is what makes it useful: it reflects what is happening inside the cell, whereas serum B12 measures a circulating pool that is largely bound to haptocorrin and not biologically available to tissues. Serum B12 in the low-normal range, roughly 150 to 350 pmol/L, is the classic diagnostic grey zone, and a raised methylmalonic acid resolves it by confirming that deficiency is functionally present.
The key distinction from homocysteine is specificity. Methylmalonic acid rises in B12 deficiency but not in folate deficiency, since folate is not a cofactor for methylmalonyl-CoA mutase. Measuring the pair therefore separates the two — a raised homocysteine with a normal methylmalonic acid points towards folate, while both raised together points towards B12.
Renal impairment is the main confounder, raising methylmalonic acid independently of B12 status through reduced clearance, which matters in older patients where both problems are common together. Small intestinal bacterial overgrowth can also raise it through bacterial production of propionate precursors, so an unexplained result with normal B12 and normal renal function is worth reviewing for gut causes.
Frequently asked questions
How do I convert methylmalonic acid from µmol/L to nmol/L?
Multiply by 1000 — the two units differ only in scale. A methylmalonic acid of 0.32 µmol/L is 320 nmol/L.
Why is methylmalonic acid better than serum B12 for diagnosing deficiency?
It reflects intracellular B12 activity directly, since B12 is a required cofactor for the enzyme that clears it. Serum B12 measures a circulating pool that is partly biologically unavailable, so it can look falsely normal.
Does methylmalonic acid rise in folate deficiency?
No. Folate is not a cofactor for methylmalonyl-CoA mutase, so methylmalonic acid stays normal in isolated folate deficiency. This is what lets the pair with homocysteine distinguish the two.
What else raises methylmalonic acid besides B12 deficiency?
Renal impairment, through reduced clearance, and small intestinal bacterial overgrowth. Both should be considered before attributing an isolated raised result to B12 status.
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References
- Stabler SP. Vitamin B12 deficiency. N Engl J Med. 2013;368(2):149–160.
- Snow CF. Laboratory diagnosis of vitamin B12 and folate deficiency: a guide for the primary care physician. Arch Intern Med. 1999;159(12):1289–1298.