Methylmalonic acid (MMA) test: when B12 results are unclear

Methylmalonic acid is a by-product of the breakdown of certain fats and proteins. Vitamin B12 is needed for the final step that clears it. When B12 is unavailable to the cells, MMA accumulates in blood and urine. That makes it a more direct sign of B12 function in the tissues than the B12 level in blood, which can look acceptable while the cells are short.

Lab Tests Online UK describes the test as measuring MMA in blood or urine and says it is primarily requested, sometimes with homocysteine, to help diagnose B12 deficiency when B12 results are equivocal.

The standard B12 test is available routinely and in most cases is enough. MMA is added in specific circumstances: when the B12 result is at the low end of normal and the person seems symptomatic, with problems such as neuropathy (numbness and tingling in hands and feet) or unexplained macrocytosis (large blood cells).

It is also useful when B12 is functionally inactivated. Lab Tests Online UK points out that intensive, frequent recreational use of nitrous oxide (laughing gas) inactivates vitamin B12, which can cause a functional deficiency and neurological damage even if B12 blood tests look normal. In such cases MMA, sometimes with homocysteine, may reveal the problem.

Occasionally MMA is requested in an acutely ill infant to look for methylmalonic acidaemia, a rare inherited metabolic disorder, in which case a random urine sample is the usual specimen.

No preparation is needed. In adults being investigated for suspected B12 deficiency, blood is the usual sample taken from a vein in the arm. Random urine is routinely requested in children being investigated for inherited metabolic disorders. Tell the clinician about any B12 injections or supplements, kidney problems or recreational nitrous oxide use, since each affects interpretation.

If MMA and homocysteine are raised and B12 is mildly decreased, an early or mild B12 deficiency may be present, indicating less B12 available at tissue level. If only homocysteine is raised, folate should be checked. If both MMA and homocysteine are normal, B12 deficiency is unlikely. Low MMA values carry no clinical significance.

Blood MMA can also be raised by kidney disease, because less is excreted in urine, and by small bowel bacterial overgrowth or dehydration. The amount of MMA does not necessarily reflect the severity of the deficiency, the likelihood of progression, or the presence or severity of symptoms, and levels can vary considerably over time.

If B12 sits at the low end of normal and you have no significant symptoms, your doctor may feel B12 is adequate and rely on that rather than a raised MMA, especially if homocysteine is normal. They may prefer to monitor over time and see how you respond to dietary changes or other treatment.

This is a matter of weighing a single laboratory marker against your symptoms, risk factors such as pernicious anaemia, bowel disease or metformin use, and the consequences of missing a treatable deficiency, which the NHS says can cause irreversible problems if left too long.

A result suggesting deficiency leads to a search for the cause and to treatment with B12 injections or tablets, as described by the NHS. A normal result lowers the likelihood of deficiency and prompts a look at other explanations for the symptoms. Ask for MMA results to be reviewed together with kidney function so that a raised value is not misattributed.