Validity of transcobalamin II-based radioassay for the determination of serum vitamin B12 concentrations.

A valid radioassay for the estimation of serum vitamin B12 in the presence of naturally occurring vitamin B12 (= cobalamin) analogues can be operated if serum transcobalamin II (TC II) is used as the binding protein. Serum samples that gave diagnostically discrepant results when their vitamin B12 content was analysed (i) by a commercial radioassay known to be susceptible to interference from cobalamin analogues, and (ii) by microbiological assay, were further analysed by an alternative radioassay which uses the transcobalamins (principally TC II) of diluted normal serum as the assay binding protein. Concordance between the results from microbiological assay and the TC II-based radioassay was found in all cases. In an extended study over a three-year period, all routine serum samples sent for vitamin B12 analysis that had a vitamin B12 content of less than 320 ng/l by the TC II-based radioassay (reference range 200-850 ng/l) were reanalysed using an established microbiological method. Over 1000 samples were thus analysed. The data are presented to demonstrate the validity of the TC II-based radioassay results in this group of patients, serum samples from which are most likely to produce diagnostically erroneous vitamin B12 results when analysed by a radioassay that is less specific for cobalamins.