Vitamin B12 deficiency can be evaluated using various biomarkers, including serum B12, active B12 (holotranscobalamin), and methylmalonic acid (MMA) levels.
Serum B12: This is the most commonly used test to screen for B12 deficiency. However, it has limitations. Serum B12 levels can be within the normal range in the presence of a functional B12 deficiency. They can also be influenced by other factors such as supplementation and folate status.
Active B12 (Holotranscobalamin): Holotranscobalamin is the biologically active fraction of B12 and makes up about 10-30% of total B12 in serum. It's the form of B12 taken up by cells in the body, hence it's sometimes a more accurate reflection of B12 status.
Methylmalonic Acid (MMA): MMA is a more sensitive and specific marker for B12 deficiency, as it increases when cellular B12 levels are low. If the body doesn't have enough B12, the conversion of methylmalonyl CoA to succinyl CoA is less efficient, leading to an accumulation of MMA.
Testing the level of MMA in the blood or urine can therefore be a useful way to diagnose a Vitamin B12 deficiency, because it's a sensitive marker of a functional deficiency at the cellular level. Elevated levels of MMA are typically indicative of a B12 deficiency, although there can be other causes as well.
This test can be particularly useful when other tests show borderline results, or to confirm a diagnosis of Vitamin B12 deficiency when symptoms are present but B12 levels appear to be within the normal range. However, interpretation of MMA tests can be complex and should be done in conjunction with other clinical and laboratory findings.
The normal range for methylmalonic acid (MMA) in the blood is typically around 0.07 to 0.27 micromoles per liter (µmol/L). However, these values may vary slightly depending on the laboratory and the specific testing methodology used.
In the context of vitamin B12 deficiency:
Blood MMA levels above 0.4 µmol/L are generally considered elevated and may suggest a B12 deficiency.
However, interpretation of these values can be complex and should always be done in the context of a comprehensive clinical evaluation. Other tests, such as serum B12, homocysteine, and a complete blood count, are also typically used to assess for B12 deficiency.