A differential white blood cell count is an important diagnostic tool. It can be done manually with a microscope and a special type of slide known as a hemocytometer. In order to do the count, a small sample a blood must be drawn. A known volume of blood is mixed with a known volume of an isotonic dilution solution. The diluted blood sample is put into the hemocytometer chamber, which has a known volume.
A hemocytometer has a grid etched into its surface to assist the technician in scanning every area of the chamber exactly once. Each white blood cell seen is identified and the number of each type encountered is recorded. These numbers can be converted into differential WBC counts via the following formula:
This calculation must be repeated for each type of white blood cell individually. For the exact procedure in step-by step form, look at this link.