Yes there is, and here is why. Measuring MMA has become recognized as the best way to determine functional B12 status at a cellular level. The reason for this is that vitamin B12 (in the form of adenosylcobalamin) acts as a co-factor for an enzyme called methylmalonylCoA mutase (MCM), which converts methylmalonyl CoA to succinyl CoA within the mitochondria of the cell which is channeled into energy production. B12 is critical for energy production and if there is not enough to meet the energy needs, then the methylmalonic acid (MMA) that is needed as the starting material for this reaction starts to accumulate in the cell and will overflow into the blood circulation. Usually we can measure a small amount of MMA in the blood, but when B12 availability is low in the cell, then this shows up as an elevated MMA level in the blood.

Now it should make sense that measuring MMA in the blood should give us a very accurate measure of the cellular B12 status, and it does, when compared to the routinely measured vitamin B12 level in blood. But is testing it in the blood, the most accurate way of measuring MMA? More on that here.