What is measured
- Serum vitamin B12: the total circulating at that moment
- Homocysteine: a functional marker that rises when B12, folate or B6 is insufficient
- Folate, where relevant, because it is read together with B12 rather than separately
Homocysteine also rises with impaired kidney function and in some genetic variants, so a high reading needs context rather than an immediate assumption that a vitamin is the cause.
How the test is done
One blood draw, and an 8 to 12 hour fast is suggested for homocysteine.
The homocysteine sample has to be separated quickly at the laboratory, because delay can raise the reading falsely. That is why this runs at a laboratory that handles it routinely rather than being shipped far.
If you already take a B12 supplement or have had a B12 injection, tell the doctor. The serum reading will be high afterwards and no longer reflects your baseline.
How results are read
A borderline serum B12 together with a raised homocysteine points more strongly than B12 on its own.
One point is often missed: high-dose folate can improve the blood picture without correcting a B12 deficiency. The anaemia appears to settle while nerve symptoms carry on. That is why B12 and folate are assessed together, not one after the other.
B12 deficiency develops slowly because the liver holds a long reserve. That is reassuring, but it also means someone can have nerve symptoms before a full blood count shows anything.
