Vitamin B12 deficiency laboratory findings
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Deficiency is defined as serum B12 levels less than 200 pg/mL (95-100% specific). Borderline B12 levels are defined between 200 and 400 pg/mL.
B12 levels are helpful, but imperfect:
Folic acid levels may not help.
Diagnosis may be especially difficult in alcoholics.
Serum B12 levels are often low in B12 deficiency, but if other features of B12 deficiency are present with normal B12 then the diagnosis must not be discounted. One possible explanation for normal B12 levels in B12 deficiency is antibody interference in people with high titres of intrinsic factor antibody. Some researchers propose that the current standard norms of vitamin B12 levels are too low. In Japan, the lowest acceptable level for vitamin B12 in blood has been raised from about 200 picograms/liter (pg/l) = 145 picomol/liter (pm/l) to 550 pg/l = 400 pm/l. 
Serum homocysteine and methylmalonic acid levels are considered more reliable indicators of B12 deficiency than the concentration of B12 in blood, see for example research at the St. Louis University. The levels of these substances are high in B12 deficiency and can be helpful if the diagnosis is unclear. Approximately 10% of patients with vitamin B12 levels between 200-400pg/l will have a vitamin B12 deficiency on the basis of elevated levels of homocysteine and methylmalonic acid.
Routine monitoring of methylmalonic acid levels in urine is an option for people who may not be getting enough dietary B12, as a rise in methylmalonic acid levels may be an early indication of deficiency.
- Methylmalonic acid: elevated in B12 deficiency (98% sensitive)
- Homocysteine: elevated in B12 and folate deficiency.
- Anti-IF Antibodies (AB): highly specific for PA, but sensitivity only 50-84%
- Anti-parietal cell ABs: less sensitive and much less specific.
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