To check for vitamin B12 deficiency, a test can be done to look at the level of B12 in the blood. A level below 148 pmol/L is considered low, and 148–221 pmol/L is intermediate.[1] The result of this test is considered a fairly accurate representation of the amount of B12 in the body. If it is low, treatment should be initiated. If it is high, or on the high end of the normal range, it is likely not a concern.
The intermediate range has become more difficult to interpret. A new understanding that a level of vitamin B12 in the lower end of the normal range could be associated with symptoms means that we now have something called subclinical vitamin B12 deficiency. In these instances, further testing might be a good idea. Testing for methylmalonic acid or homocysteine levels might help support the diagnosis of B12 deficiency if they come back as high.[2][1]