Abstract: Methods for blood sample preparation using reagent compositions which include a zwitterionic surfactant for sphering of cells to eliminate orientational noise. When the reagent composition including a dye for staining a subset of cells is reacted with a blood sample, and the reaction mixture is passed through the sensing region of a flow cytometer, the light scattered and absorbed by each cell is measured, the stained cells can be distinguished from the unstained cells, and when the cells are reticulocytes and mature erythrocytes, respectively, the volume and hemoglobin concentration of each reticulocyte or erythrocyte, and the hemoglobin content, mean cell volume, mean corpuscular hemoglobin concentration, and mean cell hemoglobin of the reticulocytes or erythrocytes are calculated from the measured cell-by-cell volume and hemoglobin concentration.