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Plasma vitamin B12 [cobalamin (Cbl)] concentrations are usually measured as a screening marker for vitamin B12 deficiency. Siemens Healthcare Diagnostics has introduced Cbl assays for various platforms, i.e., the immulite (IML) 2000 and 2500. In our laboratories, regular validation studies for the IML 2500 were conducted and showed acceptable quality specifications. After the introduction of the IML 2500 Cbl assay, clinicians in the department of internal medicine reported an increased frequency of patients with Cbl-concentrations less than 148 pmol/L.In order to investigate this claim from the clinicians, we retrospectively analyzed the internal and external quality control (QC) of the Cbl assay. In addition, the monthly patient means for the Cbl assay were analyzed both before and after the introduction of the new Cbl assay.No abnormalities were found in the internal and external QCs. However, the monthly patient means for the Cbl assay showed a statistically significant decrease in cobalamin concentrations. Siemens acknowledged the problems and formulated a new Cbl assay, which was subsequently validated in our laboratories and showed equivocal Cbl results when compared to the IML 2000 Cbl assay.We report a flawed validation study conducted by the manufacturer that resulted in an undetected analytical problem in the IML 2500 Cbl assay, its subsequent introduction on the market, the final recognition of the poor performance of the assay by our clinicians, and the eventual resolution by the manufacturer. Hence, it emphasizes the utmost importance for thorough comparison between assays over the entire measurement range, even when both assays are produced by the same manufacturer.