A microscopic view of an abnormal blood film.

A blood film or peripheral blood smear is a slide made from a drop of blood, that allows the cells to be examined. Blood films are usually done to investigate hematological problems (disorders of the blood itself) and, occasionally, to look for parasites within the blood such as malaria and filaria.

Blood films are made by placing a drop of blood on one end of a slide, and using a spreader slide to spread the blood over the slide's length. The aim is to get a region where the cells are spaced far enough apart to be counted and differentiated.

The slide is left to air dry, after this it is fixed to the slide by immersing it briefly in methanol. After fixation, the slide is stained (usually by the Romanowsky method) so the cells can be distinguished from each other.

As stated, a blood film will help identify circulating blood cells. Apart from counting the cells, morphology of cells can provide a wealth of information and assist in making a diagnosis.

Cellular components of blood are:

• Red blood cells (erythrocytes)
• White blood cells (leukocytes)
• Platelets (thrombocytes)

Normal blood films are typically full of red blood cells, with occasional white blood cells and minuscule platelets, which are harder to notice due to their size.

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).

White blood cells are classified according to their propensity to stain with particular substances, the shape of the nuclei and the granular inclusions.

• Neutrophil granulocytes usually make up close to 80% of the white count . They have multilobulated nuclei and lightly staining granules. They assist in destruction of foreign particles by the immune system by phagocytosis and intracellular killing .
• Eosinophil granulocytes have granules that stain with eosin and play a role in allergy and parasitic disease. Eo's have a multilobulated nucleus.
• Basophil granulocytes are only seen occasionally. They are polymorph-nuceated and their granules stain with alkaline stains, such as haematoxylin. They are further characterised by the fact that the granula seem to overlie the nucleus. Basophils are precursors of mast cells, the cells that mediate local allergy and immune response in organs such as the skin.
• Lymphocytes have very little cytoplasm and a large nucleus (high NC ratio) and are responsible for antigen-specific immune functions, either by antibodies (B cell) or by direct cytotoxicity (T cell). The distinction between B and T cells cannot be made by light microscopy.
• Plasma cells are mature B lymphocytes that engage in the production of one specific antibody. They are characterised by light basophilic staining and a very ecentric nucleus.
• Other cells are white cell precursors. When these are very abundant it can be a feature of infection or leukemia, although the most common types of leukemia (CML and CLL ) are characterised by mature cells, although these cells have an abnormal appearance on light microscopy (which can aid in the diagnosis).