Leukocytes or white blood cells designate all the hemoglobin free blood cells. These cells belong to the reticuloendothelial system. Based on their nuclear aspect, white blood cells can be divided into two categories: mononuclear cells and polynuclear cells. Lymphocytes and monocytes are the principal mononuclear cells and polynuclear are subdivided into neutrophilic, eosinophilic and basophilic cells. In the urinary sediment, the term leukocyte is usually interpreted as polynuclear, mostly neutrophils. The reason for this situation is that the neutrophils are, by far, the most abundant leukocytes in urine. In a normal specimen, up to 6 or 7 neutrophils / high-power field can be observed. High neutrophil counts are usually related to an inflammation process.

Mononuclear leukocytes are occasionally seen. High mononuclear counts are usually related to a high blood count pathology, infiltrating the urinary space.

Under bright field microscopy and without staining, identification of the different types of white blood cells is almost impossible. The term leukocytes is indicated for the routine microscopy. The subclasses of leukocytes can be identified by staining with the Wright or PAP stain. Efficiency of these staining procedures is highly dependent of the conservation state of the cells. Staining with these represents a workload, not justified for the routine specimen.