Unfortunately, many diagnoses of B12 deficiency are still made or dismissed based solely on a single measurement of vitamin B12 levels in the blood.

Why is that unfortunate?

Well, first of all, there's a remarkable variability between the results

obtained with different commercially available B-12 blood tests. The same sample of blood can come

in at 240 in one lab and almost double that in another. Secondly, there's significant

day-to-day variation of serum B12 within one individual.

Here, researchers tested people once a week for 10 weeks,

and the variation in each individual could be as much as 100 points or more.

One way B-12 deficiency can be missed is when someone takes a B-12 supplement right before getting tested,

so based on their blood levels,

it looks like they have more than enough, even if they're actually suffering a serious

deficiency. On the other hand, perhaps as many as one in five people who test positive

for B-12 deficiency based on low-blood B-12 levels may not actually be deficient. And then there

are those with symptomatic

B12 deficiency who test fine. As many as 5% of patients who end up being diagnosed with

vitamin B12 deficiency had blood B12 concentrations above a commonly used cutoff of about 150.

And even more had abnormal MMA or homocysteine levels, which are signs of

functional B12 deficiency, even if overt symptoms are not apparent. Levels of these metabolites

may not normalize until their blood levels get up to around 400, so maybe the traditional

cutoff is too low. And it's not just lab test abnormalities.

Those with marginal B12 levels score worse on hand dexterity tests, suggesting they

have covert diminished nerve function. Most seriously, we can have life-threatening

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