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The answer depends strictly on the tissue of interest in the research

Conclusion Whilst severe B12 deficiency has a clear clinical picture with characteristic haematological and/or neurological features, establishing mild or borderline deficiency can be problematic as serum levels are maintained at the expense of tissue concentrations, and therefore serum levels of B12 may be in the low normal or borderline range despite a deficiency

This condition, known as pernicious anemia, happens when red blood cells cant mature properly due to lack of B12

Akkus G, Sert M

Purity issues, even small ones, can throw off an entire study, so researchers tend to be picky about sourcing for good reason