As suggested by our previous RBC survival studies, we anticipate that the heaviest biotin labeling will alter the intrinsic RBC structural properties of RBCs, shortening their long term survival. The in vitro and in vivo validation studies we propose in adults are necessary because the conditions for reproducibly obtaining discrete RBC biotin densities at appropriate, closely spaced intervals have not been conclusively worked out, nor have we determined empirically which of the five densities will not artifactually shorten long-term RBC survival. We can use the lighter densities for RBC survival measurements and the heavier densities for the simultaneous RBC volume measurements needed to account for RBC volume increases caused by growth. It is important to perform these feasibility studies in both adults and infants because of the vastly different physiologic states, ie, normal, healthy adults are in steady state erythropoiesis while critically ill, anemic infants experience multiple clinical circumstances which perturb RBC survival, eg, growth, phlebotomy, intervening transfusion, etc.