AbstractIntroductionThe Thomas‐plot has proven to be a helpful tool to discriminate between different types of anemia. This plot combines the reticulocyte hemoglobin content (Ret‐He) with the soluble transferrin receptor (sTfR)/log ferritin (fer) ratio. In this study, we designed an alternative Thomas‐plot in which Ret‐He is combined with the transferrin (Tf)/log ferritin ratio. We validated both Thomas‐plots in a population of anemic patients and compared the performance to the current laboratory diagnostics of anemia.MethodsA total of 536 anemic patients were included. The first 188 patients were used to generate ROC curves to define the optimal cut‐off values for both Thomas‐plots. With the following 348 patients included we studied the performance of the alternative and classical Thomas‐plots compared to current anemia diagnostics.ResultsCut‐off values were defined (Ret‐He: 31.2 pg, sTfR/log(fer): 0.91, and Tf/log(fer): 1.71). With both Thomas‐plots the amount of e causa ignota (ECI) cases dropped from 39% to 27%. A more in depth analysis on the iron status of anemia of chronic disease (ACD) patients and a subdivision between latent and classical iron deficiencies could be made with the help of both plots. A shift from classical iron deficiency anemia (IDA) cases according to the classical Thomas‐plot toward functional IDA according to the alternative Thomas‐plot was observed.ConclusionThe alternative Thomas‐plot is an effective tool that gives a more in depth view on the iron status of anemic patients. In addition, it is easier to implement due to the use of transferrin rather than the soluble transferrin receptor.