MMA is primarily requested, sometimes along with homocysteine, to help diagnose an early or mild B12 deficiency. It may be requested as a follow-up to a vitamin B12 test result that is in the lower end of the normal range.
Some researchers have suggested using MMA as a screening tool, especially among the elderly, who frequently have B12 deficiencies and may have few recognizable symptoms. However, this use is still controversial in the medical community and only a few doctors are using MMA for this purpose. MMA testing may not be suitable for monitoring because it is subject to variation and results do not reliably trend up or down in response to B12 treatment.
Occasionally, specialised MMA testing may be requested to help diagnose methylmalonic acidaemias, a rare inherited metabolic disorder that occurs in about 1 in 50,000 to 100,000 people. Babies with this disease are unable to convert methylmalonyl CoA to succinyl CoA. They appear normal at birth but as they ingest protein, they begin to show symptoms such as seizures, failure to thrive, mental retardation, strokes, and severe metabolic acidosis. Many newborn screening programmes in the USA are now testing for this disorder, but it is not done in the UK.
Methylmalonic acid is not requested frequently. Until there are more data supporting its use and consensus on its clinical utility and long-term benefits, it will probably not be routinely used by doctors.
However, MMA may be requested, sometimes along with a homocysteine test, when a vitamin B12 test result is in the lower portion of the normal range, especially if the patient has symptoms associated with B12 deficiency. An MMA test also may be requested as a follow-up to an elevated homocysteine level if the two tests are not requested together.
Occasionally, specialised MMA testing may be requested when a doctor suspects that an acutely ill infant may have inherited methylmalonic acidaemia.
If MMA and homocysteine levels are increased and B12 concentrations are mildly decreased, then an early or mild B12 deficiency may be present. This may indicate a decrease in available B12 at the tissue level. (If only homocysteine levels are elevated, then folate concentrations should be checked). If MMA and homocysteine levels are normal, it is unlikely that there is a B12 deficiency.
Since the test for MMA is usually requested to detect elevated levels in suspected cases of B12 deficiency, decreased levels of MMA are not clinically significant.
Blood MMA levels also can be increased with kidney disease, which results in decreased MMA excretion in the urine, so MMA accumulates in the blood.
Moderately to severely elevated levels of MMA may be seen in infants with the rare inherited disease methylmalonic acidaemia.
An elevated MMA test may indicate a B12 deficiency, but the amount of MMA measured does not necessarily reflect the severity of the deficiency, its likelihood of progressing, or the presence or severity of any symptoms.
Some studies have found a high variation in MMA levels when they are measured over time.
This article was last reviewed on 22 June 2011. | This article was last modified on 10 February 2012.
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