Urinary methylmalonic acid test may have greater value than the total homocysteine assay for screening elderly individuals for cobalamin deficiency.

Se, and Se were the isotopes monitored by this technique (reaction gas, CO; sample diluted 1 24; yttrium internal standard; quantification by standard addition) (5 ). Representative selenium concentrations measured in Seronorm Level 1 and 2 serum samples are shown in Table 1. Considering the Level 1 sample, all values from the three analytical techniques agreed (within quoted uncertainty ranges) with the stated target value. Mean (SD) selenium concentrations for the Level 2 sample, however, were consistently higher than the target values: 161 (4) g/L by magnetic sector ICP-MS, 152 (5) g/L by DRC-ICP-MS, and 166 (17) g/L by ET-AAS vs the suggested target value of 136 g/L. A mean (SD) selenium concentration of 168 (17) g/L was also obtained by magnetic sector ICP-MS analysis after microwave digestion of serum samples. On the basis of our findings we propose that the selenium concentration in these batches of Seronorm Level 2 Trace Elements Serum may be inaccurate. We believe that a value of 150–165 g/L may more closely reflect the true selenium concentration in these batches. We wish to make clear that we have no reason to doubt the quality of the initial certification process or the quality of the initial calibrating laboratories. Rather, we suggest that unknown factors such as aging, contamination, or changed manufacturing practices have caused these batches of this reference sample to differ from those originally prepared and evaluated.