Serum B12 can be false normal or even false high when a deficiency is present. And a (mildly) false low value does not automatically mean a B12 deficiency.
It is generally agreed that an obviously low value means a B12 deficiency; a serum-B12 below 150 pmol/L gives a specificity of >95%.
There is a large grey area however, in which a deficiency is possible, despite a normal serum B12 value. If 200 pmol/L is used as a cut-off point, the sensitivity of serum B12 is still 75% and the specificity 72%.
One to five per cent of people, with a value above 221 pmol/L, still have a deficiency.
Serum B12 can be false normal/high in:
- Liver disease
- Too much haptocorrin (TCI) as in kidney disease and myeloproliferative diseases
- Lack of transcobalamin (TCII)
- Inborn error in the B12 metabolism
- Intestinal bacterial overgrowth
- Antibodies against TCI/TCII
- Test failure
Serum-B12 can be false low in:
- Folate deficiency
- Low haptocorrinlevels (TCI)
- Pregnancy (3rd trimester) and oral contraceptives (possibly)
- Multiple myeloma and HIV
By using serum B12 alone, many patients are being missed. Because a B12 deficiency can lead to permanent neurological damage, one should never rely on serum B12 alone (unless obviously low) and always test MMA when a deficiency is suspected. This is even more essential when neurological and/or neuropsychiatric symptoms are present. Experts in the medical literature advise to combine serum B12 or Active B12 with MMA when a deficiency is suspected.
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