AccuCheck Counting Beads are an efficient single-platform method for absolute cell counts that combines the advantages of direct flow cytometric immunophenotyping with the use of two different fluorescent beads (A and B beads). These two fluorospheres are used as a double internal standard for blood volume calculation. A known volume of AccuCheck Counting Beads is added to the same known volume of stained blood in a lyse-no-wash technique. The beads are counted along with cells. Since the concentration of beads is known, the number of cells per microliter (the absolute count) is obtained by relating the number of cells counted to the total number of fluorescent bead events. The cell number is then multiplied by the number of total fluorospheres per unit of volume. As the AccuCheck Counting Beads system contains two different fluorospheres in a known proportion, we can first assure the accuracy of the assay by verifying the proportion of both types of beads.