Blood smear

Blood film
Intervention
Blood films, Giemsa stained
ICD-9-CM	90.5
MedlinePlus	003665
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A blood film — or peripheral blood smear — is a thin layer of blood smeared on a glass microscope slide and then stained in such a way as to allow the various blood cells to be examined microscopically. Blood films are examined in the investigation of hematological (blood) disorders and are routinely employed to look for blood parasites, such as those of malaria and filariasis.

Blood films are made by placing a drop of blood on one end of a slide, and using a spreader slide to disperse the blood over the slide's length. The aim is to get a region, called a monolayer, where the cells are spaced far enough apart to be counted and differentiated. The monolayer is found in the "feathered edge" created by the spreader slide as it draws the blood forward.

The slide is left to air dry, after which the blood is fixed to the slide by immersing it briefly in methanol. The fixative is essential for good staining and presentation of cellular detail. After fixation, the slide is stained to distinguish the cells from each other.

Routine analysis of blood in medical laboratories is usually performed on blood films stained with Romanowsky, Wright's, or Giemsa stain. Wright-Giemsa combination stain is also a popular choice. These stains allow for the detection of white blood cell, red blood cell, and platelet abnormalities. Hematopathologists often use other specialized stains to aid in the differential diagnosis of blood disorders.

After staining, the monolayer is viewed under a microscope using magnification up to 1000x. Individual cells are examined and their morphology is characterized and recorded.

Characteristic red blood cell abnormalities are anemia, sickle cell anemia and spherocytosis. Sometimes the microscopic investigation of the red cells can be essential to the diagnosis of life-threatening disease (e.g. TTP).