Screening for vitamin B₁₂ Deficiency: Caveat Emptor

Gastrectomy provides a surgical paradigm for pernicious anemia; the scalpel and autoantibodies both disrupt the crucial gastric component of cobalamin (vitamin B₁₂) absorption. That patients are at risk for developing cobalamin deficiency after gastrectomy is therefore obvious and known. The time from surgery to the onset of cobalamin deficiency varies with the extent of parietal cell ablation and with the inventory of the stored vitamin that is conserved and stockpiled during times of plenty. In this issue, Sumner and colleagues [1] report the results of follow-up screening for cobalamin deficiency in a large series of patients who have had gastrectomy. By using serum vitamin B₁₂ measurements and newer laboratory assays for metabolites known to accumulate in cobalamin deficiency, Sumner and colleagues report a much higher prevalence of deficiency than that previously noted [2]. These findings raise several issues. First is the clinical implication that cobalamin deficiency may be underdiagnosed. How prevalent is true cobalamin deficiency among patients who have had gastrectomy and in the general population? Second, what are the consequences—if any—of such deficiency, and on what basis should the decision to treat such patients be made? This issue leads to the question of cost-effectiveness. Is it necessary to periodically monitor the at-risk population with serum vitamin B₁₂ and metabolite assays, or is it better simply to consign such patients to receiving monthly vitamin B₁₂ injections?

Measurement of the serum vitamin B₁₂ level has been widely used as the standard screening test for cobalamin deficiency in various demographic and disease groups. Despite its overall utility, the test has poor positive and negative predictive values because of formidable problems of sensitivity and specificity [3]. A low serum vitamin B₁₂ level frequently does not indicate cobalamin deficiency, and a “normal” serum vitamin B …