Methylmalonic Acid (MMA)
MMA is a marker of how well your body is actually using vitamin B12 at the cell level. When B12 is doing its job, MMA stays low. When B12 is low or not working properly, MMA climbs — sometimes before a standard B12 blood test shows anything unusual. If your result is above range, it is worth a conversation with your GP, who will look at it alongside your other B12 markers, your kidney health, and how you are feeling.
Targeted Testingof Australians aged over 50 have vitamin B12 deficiency — the condition an elevated MMA helps confirm.
What is methylmalonic acid (mma)?
Methylmalonic acid (MMA) is a substance your body makes as a normal by-product of breaking down certain proteins and fats. Your cells need vitamin B12 to clear MMA efficiently — when B12 is short or not working properly, MMA builds up in the blood. That is what makes it a functional biomarker (a test that reflects how well a nutrient is working, not just whether it is present) rather than a direct measure of B12 itself. Australian Prescriber describes MMA this way: it helps assess tissue-level B12 status — meaning B12 where it counts, inside your cells — when other B12 tests are borderline or inconclusive.
Why is it tested?
MMA is usually a second-line or confirmatory test — ordered when a standard serum B12 comes back borderline, or when symptoms point to B12 deficiency but the main result does not clearly confirm it. It is one of two functional markers used in this way; the other is homocysteine. Australian Prescriber also notes that if nitrous oxide exposure is suspected as a cause of B12 inactivation, MMA or homocysteine should be the first test rather than serum B12. In practice, MMA is commonly read alongside homocysteine, active B12 (holotranscobalamin — the fraction of B12 your cells can actually take up and use), and total serum B12. People at higher risk of B12 deficiency include strict vegetarians and vegans. Others at risk include those with malabsorption conditions such as pernicious anaemia (where the gut cannot absorb B12 properly) or coeliac disease. People on long-term metformin or proton-pump inhibitors, and older adults, are also more likely to be tested.
Reference range (Australia)
There is no established sex difference in MMA, but levels rise with age and with reduced kidney function, so reference intervals are laboratory- and age-specific. Use the range on your own report. In published population data the upper limit sits in the region of 270–300 nmol/L for adults under 65 with normal kidney function, and higher for older adults; no harmonised Australian interval was confirmed. Some laboratories report in µmol/L (1 µmol/L = 1000 nmol/L), so check the unit on your report.
Reference intervals vary between laboratories. Always use the range printed on your own report.
What your result means
A low or in-range MMA is reassuring. Australian Prescriber notes that normal MMA and homocysteine concentrations effectively exclude vitamin B12 deficiency. A result below the reference range carries no separate concern. Your GP reads it alongside your overall B12 picture.
An in-range MMA suggests that B12 is being used adequately at the cellular level, which is a good sign. No specific follow-up related to MMA is usually needed unless you have ongoing symptoms your GP is still investigating.
An above-range MMA suggests your cells may not be using B12 efficiently — what is called functional B12 deficiency — and this can show up even when your serum B12 looks borderline adequate. Australian Prescriber notes that elevated MMA may indicate functional B12 deficiency. There is also an important caveat: reduced kidney function can raise MMA on its own, without any B12 problem at all. Your GP weighs the result against your kidney health, your other B12 markers, your symptoms, and your history before drawing a conclusion. It is not able to confirm a deficiency on its own — worth raising at your next visit, or sooner if you have symptoms that are bothering you.
What can affect your result
The biggest driver of MMA is how well your body is using vitamin B12. Anything that depletes B12 or stops your gut absorbing it will push MMA up. That includes pernicious anaemia (an autoimmune condition where the stomach cannot produce the protein needed to absorb B12), coeliac disease, Crohn disease, bariatric surgery, a strict vegan or vegetarian diet, and long-term use of medicines such as metformin or proton-pump inhibitors. Kidney function is the other major factor. Reduced kidney function impairs the body's ability to clear MMA from the blood and can raise it entirely independently of B12. Research shows that B12, kidney function, age, and sex together account for only about a fifth of the variation in MMA between people — other undetermined factors play a role too. One thing MMA has going for it: unlike homocysteine, it is not affected by folate deficiency, which makes it a more B12-specific signal. Rare inherited enzyme disorders can also raise MMA, though these are uncommon.
When to act
An elevated MMA is worth a conversation with your GP — not a reason to panic, but a useful prompt to look more closely at your B12 picture. They will read it alongside your serum B12, active B12 (holotranscobalamin), and homocysteine, and will factor in your kidney health, your symptoms, and any risk factors such as diet or long-term medication use. If your MMA is in or below the reference range, no specific action related to MMA is usually needed. Your GP may keep investigating if other B12 markers are also out of range or if you have ongoing symptoms that have not been explained. This page is general information and is not a substitute for advice from your GP.
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