Urinary MMA or serum MMA can pick up on a deficiency much earlier than serum B12. Additionally, a while back I got to view a series of tests where individuals had elevated serum B12 while the MMA indicated B12 deficiency. Here is an example:
Note: High MMA indicates B12 deficiency, The MMA-Normalized is the marker to pay attention to.
Although creatinine was low in the labs above, the chem screen showed normal kidney function.
What is interesting is that I had visibility to follow up in 3 of these cases and the addition of adenosylcobalamin normalized the MMA. These individuals were only taking high doses of methylcobalamin.
You can see in the image below that adenosylcobalamin is responsible for lowering MMA. Theoretically, any form of B12 you take should be able to convert intracellularly to other forms, however, case reports on this forum and in the 3 individuals mentioned above indicate some individuals may need to supplement with adenosyl.