Undetected vitamin B12 deficiency due to false normal assay results.

Cobalamin metabolism is complex and involves a series of processes any of which, if absent, may lead to cobalamin deficiency. The main causes of cobalamin deficiency are food-cobalamin malabsorption (53%), pernicious anaemia (33%), insufficient nutritional vitamin B12 intake (2%) and post-surgical malabsorption (1%)1,2. Dietary causes of the deficiency are limited to elderly people who are already malnourished or to strict vegetarians while malabsorption occurs in patients suffering from several gastrointestinal conditions3. Vitamin B12 is an important co-enzyme required for tetra-hydrofolate production which itself is necessary for normal DNA synthesis. B12 deficiency, therefore, results in impaired DNA formation. The consequent slowing down of cell division leads to the formation of megaloblastic cells especially of those cells with a rapid turnover such as haematopoietic cells and intestinal epithelial cells1. Although measurement of vitamin B12 levels is the gold standard for the diagnosis of B12 deficiency some reports do exist concerning difficulties in its assay4–8. During the 1970s and 1980s vitamin B12 was measured using a 57Co-based radioisotope; since the 1990s, however, with the introduction of automated analysers, most methods are based on solid-phase competitive chemiluminescence enzyme immunoassays9,10. The principal problems with these more recent assays are caused by the presence of intrinsic factor antibodies and heterophilic antibodies in the test sample. Obviously, this is a significant limitation to B12 assays in pernicious anaemia and raises the need to find more sensitive and specific tests to confirm vitamin B12 deficiency11,12. Despite these specificity restrictions and controversy about sensitivity, measurement of plasma cobalamin is still the gold standard for diagnosing vitamin B12 deficiency and its determination can provide intriguing, but misleading information in clinical practice13.