Methylmalonic acid (MMA) test: what it measures, normal levels, and why a high MMA means functional vitamin B12 deficiency

Every cell breaks down certain fats and amino acids through a step that converts methylmalonyl-CoA to succinyl-CoA. The enzyme responsible, methylmalonyl-CoA mutase, requires a form of vitamin B12 as its cofactor. When B12 is short inside cells, that step slows, methylmalonyl-CoA backs up, and it spills into the blood and urine as methylmalonic acid. The MMA test measures that spillover — a direct readout of whether B12 is actually working at the tissue level.

That is why MMA is called a functional marker. The standard serum B12 test measures how much of the vitamin is circulating, most of it bound to a carrier protein that cells cannot use. Serum B12 can sit in the low-normal range (roughly 200–300 pg/mL) in a person whose cells are genuinely deficient, and it can read normal or high in someone taking supplements whose absorption is impaired. MMA cuts through that: if MMA is high, the cells are not getting enough usable B12, whatever the serum level says.

MMA is measured by mass spectrometry in serum or plasma and reported in nanomoles per liter (nmol/L); some labs use micromoles per liter (1 µmol/L = 1,000 nmol/L). A urine MMA test exists and is used mainly in newborn screening for the inherited disorder methylmalonic acidemia, a separate condition from nutritional deficiency.

Reference intervals vary more than for most tests because laboratories set them from different populations and methods. Labcorp reports 0–378 nmol/L; Quest's interval is approximately 87–318 nmol/L; research studies commonly use thresholds of 270–400 nmol/L to define metabolic B12 deficiency. A result should be read against the interval printed on the report, and repeat tests compared only within the same laboratory.

A low or undetectable MMA has no clinical significance — it simply indicates that B12-dependent metabolism is running normally. The test is interpreted at its upper end only.

MMA is not a first-line screen; serum B12 (often with folate) comes first. MMA is added when the B12 result is borderline (roughly 200–300 pg/mL) and the question is whether it matters, when someone has symptoms that fit B12 deficiency — numbness or tingling, balance problems, memory change, fatigue, a sore tongue, macrocytic anemia — despite a normal B12, or when a baseline is needed in someone already taking B12 supplements, whose serum level is no longer informative.

It is also used in groups at high risk of malabsorption: people who have had gastric bypass or other stomach or ileal surgery, people with Crohn's disease, celiac disease or atrophic gastritis, long-term users of metformin or acid-suppressing drugs (proton-pump inhibitors, H2 blockers), vegans and strict vegetarians, adults over 65, and people with pernicious anemia who want to confirm their treatment is adequate.

Functional B12 deficiency is the cause the test exists to find, but several other factors raise MMA and have to be considered before concluding that B12 is the problem:

Homocysteine is the other metabolite that accumulates in B12 deficiency, but it also rises with folate deficiency, vitamin B6 deficiency, kidney disease and certain genetic variants. Measuring both sharpens the picture: a high MMA with a high homocysteine points to B12 deficiency; a normal MMA with a high homocysteine points to folate or B6 rather than B12; a high MMA with a normal homocysteine suggests B12 deficiency or a kidney effect. Adding serum B12 and a creatinine/eGFR to the same draw resolves most ambiguous cases without a second visit.

Some laboratories offer an active B12 (holotranscobalamin) test, which measures the fraction of B12 actually available to cells. It is another way of addressing the same borderline-B12 problem; MMA remains the more widely available and better validated option in the United States.

A raised MMA with a low or borderline B12 is treated as B12 deficiency. Replacement — by injection or high-dose oral or sublingual B12, a choice the clinician makes based on the cause and the severity — corrects the metabolic block, and MMA falls within weeks, usually normalizing by two to three months. A repeat MMA at about three months confirms the response; a persistently high level despite treatment prompts a look at kidney function, adherence and absorption.

Finding the cause matters as much as replacing the vitamin. Depending on the history that may mean intrinsic factor antibodies (pernicious anemia), a review of metformin or acid-suppressant use, celiac serology, a dietary history, or asking about nitrous oxide exposure, which inactivates B12. Folate is checked at the same time because the two deficiencies travel together and folate alone can mask B12 deficiency's blood-count changes while the nerve damage continues.

MMA is a blood draw with no special preparation; fasting is not required, though some laboratories prefer it and it makes repeat results more comparable. Because it is a specialty mass-spectrometry assay, results take longer than routine chemistry — often about a week. You can have it ordered by a clinician or directly through TestWell, where the sensible companion tests are serum B12, folate, homocysteine and a metabolic panel for creatinine and eGFR, all from the same draw. Stay well hydrated before the visit, and note any B12 supplements or injections on the request, since they change what a given result means.