The total count of circulating WBCs is differentiated according to the five types of leukocytes, each of which performs a specific function.

The differential count is expressed as a percentage of the total number of leukocytes (WBCs). The distribution (number and type) of cells and the degree of increase or decrease are diagnostically significant. The percentages indicate the relative number of each type of leukocyte in the blood. The absolute count of each type of leukocyte is obtained mathematically by multiplying its relative percentage by the total leukocyte count. The formula is: