Methods for measuring a 6 log10 white cell depletion in red cells

Methodology is presented for enumerating very low concentrations of white cells (WBCs) in red cells (RBCs) by two separate measurement techniques. Both techniques rely on the method of harvesting WBCs from a 300- to 350-mL unit of RBCs and concentrating them to a volume of approximately 0.5 to 1.0 mL, which is equivalent to a WBC concentration of approximately 550 to 1. The WBC separation and concentration steps require less than 3 hours to complete, and multiple RBC units can be processed in parallel. Cell counting is carried out in a fluorescence hemocytometer or by a modified cytospin technique. As few as 1000 WBCs in a unit of RBCs, which corresponds to a more than 6 log10 WBC depletion, can be measured without reaching the sensitivity limit of either technique (800 and 200 WBC/unit, respectively). The harvesting method and counting techniques are relatively simple and inexpensive.