Differential Count of White Blood Cells

The differential WBC count is the laboratory method used to determine the relative proportion of each type of white blood cell present in the blood, by carefully examining a well-stained peripheral blood smear (prepared as described above) and counting cells one type at a time. The number of each distinct white cell type found is then expressed as a percentage of the total number of white cells counted, rather than as an absolute count, which makes the result easy to compare against standard reference ranges. Five main types of leucocyte are distinguished and counted this way: neutrophils, which normally make up the largest share of white cells at about 50-70%; lymphocytes, the second most abundant at roughly 20-40%; monocytes, at about 2-8%; eosinophils, at about 1-4%; and basophils, the rarest type, at about 0-1%. Because infection, allergy, parasitic disease and several blood disorders each shift this balance in a characteristic way - for example, a raised eosinophil percentage often points toward an allergic or parasitic condition, while a raised neutr …