A rose is a rose is a rose, or not.

In this issue of Blood, Roubinian et al provide important evidence to confirm, and refute, a long-standing maxim in clinical medicine that a 1-unit transfusion of red blood cells (RBCs) should yield a posttransfusion hemoglobin increment of 1 g/dL. Although true, in general, this rule was not always accurate, and deviations could be misleading. They evaluated many single-unit transfusion outcomes in stable adult patients by mining electronic health records (EHRs) and linked blood donor data. This approach was not only pragmatic, but it also points the way to future studies.