The methylmalonic acid (MMA) serum test is used to help diagnose the early onset or mild B12 deficiency. Researchers suggest using MMA as a screening tool in elderly patients who often have B12 deficiency but do not have discernible symptoms. It is worth noting that this is still controversial because the results do not reliably trend up or down in response to B12 treatment. It may be ordered along with a homocysteine test when a vitamin B12 test is in the lower part of the normal range. If MMA and homocysteine levels are increased and the level of vitamin B12 is mildly decreased, there may be early or mild deficiency present. However, increased MMA levels alone may indicate a folate deficiency instead. A decreased level of MMA is not common and not considered to be significant to a patient’s health.