Differential Counts

The magnitude and sources of variation in the white blood cell (WBC) count and dif-ferential count affect their information content and clinical value. This study describes components of variation in the WBC count and differential, estimates the magnitude of each component, and uses computer simulations to compare the infor-mation conveyed by the total WBC count and by the WBC differential count, expressed as the number of each type of cell and as the percentage of each cell type. Biologic variation is much greater than statistical sampling variation in differen-tial WBC counts, even when a relatively small number of cells is classified. The com-monly reported neutrophil percentage is inferior both to the neutrophil count and to the total WBC count in its ability to detect neutrophilia and to detect recovery from elevated levels. This conclusion holds for single as well as for sequential WBC differ-ential determinations and regardless of the level of test result at which the clinician considers disease to be present. The total WBC count and the neutrophil count differ little in performance, so a neutrophilic patient’s return to normal levels can safely be detected and monitored by relying on the less expensive total WBC count and for-going repeated requests for differential counts. (Med Decis Making 4:69-80, 1984