Measurement of kynurenine levels in 13 091 donors from the REDS RBC Omics index cohort. (A) Kynurenine measurements were performed on 13 091 end-of-storage (day 42) pRBC units from the REDS RBC Omics index donor cohort. (B) Kynurenine levels were found to be nonnormally distributed in this population. (C) 3D uMAP of the 13 091 donors, based on metabolomics analyses (informing uMAP1 and uMAP2) and kynurenine levels (z-axis and color scheme). This analysis highlights a subset of donors with significant elevation in kynurenine levels compared with the rest of the population. (D) Line plots show kynurenine levels (y-axis) as a function of donor age (x-axis), either alone (top) or as a function of donor sex (middle) or BMI (bottom). (E) Kynurenine levels rank among the top correlates to osmotic fragility in the index donor cohort. (F) Correlation analyses were performed for end-of-storage (day 42) kynurenine levels of the 643 donors who were screened both at index and recalled donation (2 independent blood units). Results indicated a significant level of reproducibility for kynurenine within the same donor across multiple donations in panel F. Kyn, kynurenine.