A blood smear allows the evaluation of white blood cells, red blood cells and platelets. These cell populations are created by and matured in the bone marrow and released into the bloodstream as required. WBC or White Blood Cell is used by the body to fight infection, RBC or Red Blood Cell to carry oxygen to the different organs of the human body and Platelets or Thrombocytes to help clot the blood. The number and type of each cell present in the blood is dynamic, maintained by the body within normal ranges, but fluctuating slightly all the time, depending on the needs of the body. A peripheral blood smear is a snapshot of the cells that are present in the blood at the time that the sample is obtained. To create a blood smear, a single drop of blood is spread in a thin layer across a glass slide, dried, and then stained with a special dye. Once the stain has dried the slide is evaluated under a microscope by laboratory personnel.

     The drop of blood on the slide contains thousands of red blood cells, hundreds of white blood cells and numerous platelets. Along the leading feathered edge of the smear the blood cells are present on the slide in a single layer. Under the microscope they can be plainly seen, allowing the microscopist to roughly estimate the number of each type of cell present, and to compare their size, shape, and general appearance to established normal cells, based on its appearance on the slide or smear. It is possible to distinguish between the different types of WBCs and to determine their relative percentages by counting 100 of them. The examination can also be used to evaluate the size, shape, and hemoglobin content of the RBCs, and to determine whether there are an adequate number of typical looking platelets present. A blood sample is obtained by inserting a needle into a vein in the arm or by pricking a finger, ear, or in the case of an infant, a heel. Laboratory examination of a sample of blood is done to obtain information about its physical and chemical properties. Blood analysis is commonly carried out on a sample of blood drawn from the vein of the arm, the finger, or the earlobe; in some cases, the blood cells of the bone marrow may also be examined. Hundreds of hematological tests and procedures have been developed, and many can be carried out simultaneously on one sample of blood with such instruments as autoanalyzers. Blood analysis includes the following areas of study in determination of the number of red blood cells, classification of blood and study the structure of blood proteins and determination of the activity of enzymes. Other properties of blood that may be included in an analysis are total volume, circulation time, and viscosity, clotting time and clotting abnormalities, acidity, level of oxygen and carbon dioxide, and clearance rate of various substances. In addition to the wide variety of procedures devised for the study of normal blood constituents, there are also special tests based on the presence in the blood of substances characteristic of specific infections, such as the serological tests for syphilis, hepatitis, and human immunodeficiency virus.