Reproducibility of metabolite levels across 2 independent donations in 643 repeated blood donors from the REDS RBC Omics study. (A) Two distinct packed RBC units were donated by 643 donors and stored for 42 days, before metabolomics analyses. Metabolomics measurements on both index (first unit) and recalled (second) blood units from the same donors were correlated to test whether metabolite levels were reproducible at the end of storage across multiple donations from the same donor. (B) Results indicate a right-shifted correlation curve, because 80% of the measured metabolites showed significantly (P < .05) reproducible (ie, positively correlated) metabolite measurements across donations. (C) Over-represented among the top 20 most significantly reproducible metabolites, we identified multiple carnitine precursors and AC conjugates. (D) Pathway analysis of the top 50 most reproducible metabolites identified carnitine synthesis and metabolism as the most reproducible pathway across donations. (E) Representative metabolites in this pathway are shown, numbered according to their order of appearance in panel C. (F) Representative scatter plots showing matched index vs recalled unit measurements for representative small molecules in the carnitine synthesis and metabolism (Spearman rho and P value for these correlations are noted within each panel).