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Vitamin B12 deficiency can be caused by many factors, 1 of which is pernicious anemia, a condition resulting in deficient production of intrinsic factor in the parietal cells of the stomach. Intrinsic factor is a protein that is needed to assist in the absorption of vitamin B12 into the small intestine. Vitamin B12 is converted into adenosylcobalamin, which converts L-methylmalonic acid to succinyl coenzyme A; hence, a decrease in vitamin B12 absorption in the intestine can cause an excess of methylmalonic acid within the body.
Vitamin B12 deficiency may present with any combination of the following: macrocytic anemia, glossitis (painful inflammation of the tongue), peripheral neuropathy, weakness, hyperreflexia, ataxia, loss of proprioception, poor coordination, and/or affective behavioral changes. These manifestations may occur in any combination; many patients present with neurologic symptoms without macrocytic anemia.
A group of tests is often required to establish the correct diagnosis as determination of vitamin B12 in serum does not detect all cases of vitamin B12 deficiency. Mayo Clinic's Department of Laboratory Medicine and Pathology offers a diagnostic algorithm to expedite the identification of patients with vitamin B12 deficiency. This algorithm takes into account the following facts:
-The most sensitive test for vitamin B12 deficiency at the cellular level is the assay for methylmalonic acid (MMA).
-Nearly half of the cases of pernicious anemia can be unambiguously identified if the serum test for intrinsic factor blocking antibody is positive (this is a simpler and less expensive test than the MMA).
-Serum gastrin is usually markedly increased in pernicious anemia (as a result of gastric atrophy) and this test can be used as a substitute for the more complicated and more expensive Schilling test of intestinal absorption of vitamin B12.
The algorithm is similar to that published by Green,(1) except that the serum gastrin assay is performed in place of the Schilling test. Experience with both Mayo Clinic and Mayo Medical Laboratories' cases has corroborated that this is a cost-effective alternative to the Schilling test.
In our experience, >90% of laboratory test costs can be saved by using the algorithm rather than ordering all of the services for a patient suspected of having B12 deficiency. Furthermore, the substitution of the serum gastrin assay for the Schilling test offers 3 advantages:
1. It is an in vitro test that does not require administration of radioisotopes to patients
2. It can be performed on mailed-in specimens
3. It is much less expensive
Only those tests that are appropriate, as defined by the algorithm, will be performed.
Diagnosis of pernicious anemia
Diagnosis of vitamin B12 deficiency-associated neuropathy
Vitamin B12 >400 ng/L
Results do not suggest B12 deficiency-no further testing.
Vitamin B12 150 to 400 ng/L
Borderline vitamin B12 level-methylmalonic acid (MMA) is performed. If MMA is >0.40 nmol/mL, then intrinsic factor blocking antibody (IFBA) is performed.
Vitamin B12 <150 ng/L
Vitamin B12 deficiency-IFBA is performed. If IFBA is negative or indeterminate, then gastrin is performed.
MMA < or =0.40 nmol/mL
This value implies that there is no vitamin B12 deficiency at the cellular level.
Consistent with pernicious anemia, Graves disease, or Hashimoto thyroiditis.
Gastrin >200 pg/mL
Result consistent with pernicious anemia.
Gastrin <200 pg/mL
Result does not suggest pernicious anemia.
See Vitamin B12 Deficiency Evaluation in Special Instructions.
See individual test listings.
1. Green R, Kinsella LJ: Current concepts in the diagnosis of cobalamin deficiency. Neurology 1995;45:1435-1440
2. Lahner E, Annibale B: Pernicious anemia: new insights from a gastroenterological point of view. World J Gastroenterol. 2009 Nov 7;15(41):5121-5128