Sera obtained after transfusions of older RBCs enhance proliferation of a bacterial pathogen in vitro. (A) Bacterial growth of E coli in serum samples obtained after fresh or older RBC transfusions was determined by serial optical density measurements at 600 nm for up to 5 hours after inoculation. Each point in the graph represents the mean ± SEM of the area under the curve (AUC) of the resultant bacterial growth curve (N = 14 paired values). (B) A Pearson correlation was used to determine the relationship between the mean difference in bacterial growth between fresh and older RBC transfusions at each time point and the corresponding differences in plasma non–transferrin-bound iron levels. The P values are as specified in the figure.