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Answering your questions; low B12 determination.

Q I have a physician telling me that all our B12 determinations are low. The low B12 result does not agree with the clinical picture. What are some of the pitfalls in the collection, storage, and transport of specimens that would cause a loss of B12?

A The primary uses of vitamin B12 measurements are in the investigation of patients with unexplained anemia, those with motor and/or sensory neuropathies, and patients with unexplained neuropsychiatric symptoms. A low vitamin B12 can be due to a number of causes including malabsorption, intrinsic factor antibodies, insufficient dietary intake, and autoimmune disorders. Low vitamin B12 concentrations are not necessarily indicative of deficiency. In serum, most vitamin B12 is bound to transcobalamin I, which is released by granulocytes. Low B12 concentrations may be due to a reduction in granulocyte mass and have been observed in conditions where the bone marrow is hypoplastic, aplastic, or replaced by malignant cells.

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The effect of age on vitamin B12 levels is controversial. Some reports suggest that concentrations decrease with age, while others show no age-related changes. In any event, reference intervals are method-dependent, and each laboratory should establish its own reference intervals.

The type of sample collected for vitamin B12 determinations can affect measured levels. Serum or plasma collected in EDTA has been found to be acceptable. Heparin has been found to be able to bind the vitamin, so heparin plasma should not be used. Also, the lithium salt of heparin may stimulate the release of B12 binding proteins from granulocytes, which may interfere with some assays. Use of serum separator tubes may be associated with spuriously elevated serum vitamin B12 concentrations in the Bayer Centaur assay. (1) Laboratories receiving samples for vitamin B12 assay in serum separator tubes that have already been centrifuged should either re-centrifuge the tubes, or aliquot and re-centrifuge serum from the tubes prior to vitamin B12 assay. The effect of sunlight on vitamin B12 concentrations in plasma is controversial. Contradictory studies have been published describing the stability of vitamin B12 in serum exposed to sunlight. (2) Until this is sue is resolved, shielding specimens from direct sunlight may be appropriate.

References

(1.) Lowrey I, Smith G. Elevated results in a vitamin B12 assay when using serum separator blood collection tubes. Annals of Clin Biochem. 2003;40:560-562.

(2.) Burtis CA, Ashwood ER, eds. Tietz Textbook of Clinical Chemistry. 3rd ed. Philadelphia, PA: WB Saunders. 1999;1690-1693.

--Steven C. Kazmierczak, PhD (DABCC)

Department of Pathology

Oregon Health and Science University

Portland, OR

Edited by Daniel M. Baer, MD

Daniel M. Baer, MD, is professor emeritus of laboratory medicine at Oregon Health and Science University in Portland, OR, and a member of MLO's editorial advisory board.
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Author:Kazmierczak, Steven C.
Publication:Medical Laboratory Observer
Geographic Code:1USA
Date:Dec 1, 2006
Words:453
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