Measuring and Interpreting Holo-Transcobalamin (Holo-Transcobalamin II)

Extract

At one time, diagnosing cobalamin deficiency was fairly simple, usually involving a patient with clinical problems, and was usually settled by determining whether the serum cobalamin concentration was low or not. As reviewed elsewhere (1), things began to change after sensitive metabolic tests were introduced and as attention extended to the cobalamin status of asymptomatic persons. Metabolic studies confirmed that most low cobalamin concentrations in asymptomatic patients and seemingly healthy persons represented subclinical cobalamin insufficiency, but ∼30–40% of these cobalamin concentrations did not represent insufficiency and thus could be considered “falsely low”. The diagnostic reliability of serum cobalamin was further challenged by metabolic demonstrations of “falsely normal” cobalamin concentrations; these too occurred most often, but not exclusively, in asymptomatic persons.

The search continues for the optimal test to diagnose deficiency because the metabolic tests also have disadvantages. Increased plasma total homocysteine is too nonspecific; methylmalonic acid determination, although posing fewer problems of specificity, is complex and expensive; and the deoxyuridine suppression test is too unwieldy for practical use.