The functional quality of transfused blood depends upon oxygen delivery to the tissues. Oxygen carrying capacity is easily and routinely measured using hemoglobin assays, but the kinetics of oxygen unloading from RBCs is more difficult to measure, especially in a blood bank setting. Researchers, however, recently found a simple proxy to estimate the time constant of oxygen-unloading (τ). Since cell shape relates to the metabolic state of RBCs—fresher RBCs are biconcave with wider side-scatter compared to more spherical, older RBCs—researchers found that oxygen unloading could be estimated using a ratio between side and forward light scattering data routinely collected on hematology analyzers. This ratio referred to as FlowScore is able to predict with greater than 80% sensitivity and specificity freshly drawn blood from expired blood. FlowScore was verified using data from four national blood-banking systems in Australia, Canada, England and Spain. Moreover, biochemical rejuvenation of stored RBCs restored FlowScore values to the fresh reference range. Based on two large data sets (COMPARE and LifeLines), FlowScore increased with age, was lower in females, and was affected by smoking. FlowScore is a cost-effective, accessible surrogate of oxygen unloading from RBCs and may be used as a proxy of the functional quality of blood.
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