Objectives: The 'Thomas plot' is a very helpful diagnostic tool for evaluation, monitoring and therapy of the iron status and on the hemoglobinization of the reticulocytes of patients. In 2021 Roche Diagnostics launched a second generation assay for determination of the soluble transferrin receptor (sTfR). Here we compare the old and the new assay for sTfR and analyze the consequences for the 'Thomas plot'.Methods: Measurement of sTfR, ferritin and CRP were done using a Cobas8000 system. Hemoglobin content of reticulocytes (Ret-He) was determined using a Sysmex XN9000 system.Results: The second generation of sTfR assay showed consistently lower sTfR values compared to the first generation, which would result in a left shift of the 'Thomas plot' and may lead to false diagnosis of patients using the original cut-offs. Fifteen thousand five hundred ninty two data sets for 'Thomas plot' from 2016 to 2021 were retrospectively analyzed to estimate how many patients in our hospital would be affected. In result around 5 % of all 'Thomas plots' would be affected by the lower sTfR values of the second generation assays.Conclusions: Due to the lower sTfR values measured with the second generation assay new cut-offs for the Ferritin-Index (sTfR/lg Ferritin) should be used in order to correctly diagnose the iron status of patients.