The blood smear, blood film is a method for the preparation of whole blood for microscopic examination. It is used for simple cytological or hematological assessment of blood cells (differential blood picture) and the detection of bacteria, filaria, plasmodia and other parasitic pathogens in peripheral blood. If one puts whole blood directly on a slide, the density of the red blood cells (erythrocytes) is usually so high that a differentiated assessment of the white blood cells (leukocytes) is not possible.