Storage under hypoxia improves the ability of red cells to release oxygen in ex vivo–perfused human kidneys

• Hypoxic storage of red blood cells improves oxygen diffusion capacity and respiratory rate in ex-vivo perfused human kidneys. • The beneficial effect of hypoxic storage on red cell function can be monitored using FlowScore, a flow-cytometric surrogate of cell shape. Stored red blood cells (RBCs) undergo functional attrition that manifests as a slowing of oxygen release. Previous studies demonstrated how storage under hypoxic conditions can mitigate this deterioration, but whether this improves oxygen delivery to tissues is unknown, yet critical for understanding transfusion outcomes. We tested this by perfusing human kidneys with RBCs stored either conventionally or under hypoxia. Storage under hypoxia reprogrammed RBC metabolism: accelerated glucose depletion and lactic acid production, and preserved higher ATP levels for longer. Changes in RBC shape were tracked by flow-cytometric light scattering and summarized as FlowScore, an algorithmic surrogate of the storage lesion that correlates with oxygen release kinetics. Hypoxic storage delayed the adverse rise in FlowScore by ∼3 weeks, defining a window for ex vivo kidney testing. Split blood units were stored conventionally or under hypoxia, and perfused when donor kidneys became available at ∼3 weeks. Because renal blood flow governs both oxygen supply and tubular transport workload, it allowed control of renal oxygen demand. Switching from conventional to hypoxic stored RBCs increased renal respiratory rate by ∼20% and oxygen diffusion capacity, a measure of the barriers to oxygen transfer, by ∼70%. Since tissue properties are unaltered during perfusion switches, the enhanced oxygen transfer is attributable to improved RBC function. These data show that storing RBC under hypoxic conditions can bring a measurable organ-level advantage in oxygen delivery, demonstrated in normothermically perfused kidneys. This links metrics of RBC quality with organ-level oxygen handling, and motivates further investigations on transfusion outcomes.