Also known as “reticulocyte harvest,” this is a specialized procedure performed mostly in reference laboratories that allows blood bank workers to distinguish native RBCs from transfused RBCs in a recently transfused patient. Reticulocytes are “young” RBCs that have been recently released from the bone marrow (see an image here), and they are usually present in increased quantities in anemic patients. Reticulocytes are larger and less dense than mature RBCs, and while they are pretty easily identified on a peripheral smear, that fact doesn’t really help in the reference lab when you are staring at a tube of blood! However, in the 1980’s, researchers discovered that when multiple capillary tubes of blood from a recently transfused patient are centrifuged at high speed, the least dense cells (i.e., the ones at the TOP of the capillary tube) are primarily reticulocytes, and those reticulocytes are primarily from the patient rather than the donor(s). The RBCs may then be used to identify the antigens carried on the patient’s own RBCs. This technique works pretty well, though it isn’t perfect. Generally speaking, at least 3 days should have passed since the most recent transfusion or some of the less mature transfused RBCs could contaminate the patient reticulocyte population. In addition, if the patient has a disease that results in a suppressed reticulocyte response, then there may be insufficient native reticulocytes present for the procedure to work. Most reference labs, and some hospital transfusion services, use RBC antigen molecular genotyping (approved by the FDA in the US) instead of reticulocyte separation, as genotyping is unaffected by recent transfusion.