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Peripheral Blood Smear

A peripheral blood smear or a blood film refers to a thin layer of blood that is smeared on a microscopic slide, which is then stained in such a manner so as to permit the different blood cells to be investigated microscopically. Peripheral blood smears are generally examined to explore hematological issues or conditions of the blood and also on occasions, to check for the presence of different parasites such as filarial or malaria within the blood.

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The preparation of a peripheral blood smear

A peripheral blood smear is prepared by putting a drop of blood on one side of a slide and then employing a spreader slide to scatter the blood across the length of the slide. The main purpose is to get an area, known as a monolayer, where the cells can be found to have enough individual space so as to facilitate their differentiation and counting. The monolayer is present in the ‘feathered edge’ made by the spreader slide as it pushes the blood in the forward direction.

The slide is then kept to air dry, after which the sample of blood is attached to the slide by dipping it in methanol for a short time. This affixing process is important for better staining and appearance of the cellular detail. After the blood is fixed onto the slide, the device is stained to differentiate the cells from one another.

The scheduled analysis of blood in different laboratories is generally carried out on peripheral blood smears stained with Wright’s, Romanowsky, or Giemsa stain. The combination of Wright-Giemsa stain is another popular option. Such stains permit the discovery of various abnormalities in the different components of blood such as the red blood cells, the white blood cells, the platelets, etc. There a number of other specific stains which are often used by hematopathologists to help in the differential diagnosis of blood diseases.

Once the staining is done, the monolayer is examined under a microscope that comes with magnification of up to 1000x. Individual cells are viewed and their morphology is distinguished and recorded.

When is a peripheral blood smear ordered?

A peripheral blood smear is typically ordered to assess populations of blood cells when a complete blood count with differential, which is carried out with an automatic blood cell counter, specifies the occurrence of immature or abnormal cells. It may also be conducted when a physician suspects a disorder, disease or deficiency that is causing excessive cell damage or decreased production of blood cells, which include myeloproliferative disorders, anemia, leukemia or bone marrow disorders.

A peripheral blood smear may also be performed on a regular basis as part of a patient’s monitoring or treatment for a blood cell associated condition.

Examination of blood via peripheral blood smears

  • Some of the known abnormalities associated with red blood cells include conditions such as spherocytosis, sickle cell anemia and anemia. Occasionally, the microscopic evaluation of the RBCs can be vital to the detection of a life threatening medical condition
  • The WBCs are categorized as per their tendency to stain with specific materials, the granular inclusions and the shape and size of the nuclei
  • Neutrophil granulocytes generally form up to eighty percent of the WBC count. They consist of lightly staining granules and multilobate nuclei. They aid the process of foreign matter destruction by the immune system via intracellular killing and phagocytosis.
  • Basophil granulocytes are rarely found. These are polymorphonucleated and alkaline stains like haematoxylin cause their granules to stain dark. Further features of basophils include the possible covering of the nucleus by the granules. They are comparable if not equal in cell lineage to mast cells. However, research on this similarity still continues.
  • Plasma cells are considered as mature B lymphocytes that aid the process of producing one particular antibody. They feature a very peculiar nucleus and light basophilic staining
  • Eosinophil granulocytes play a part in parasitic disease and allergies. They have granules that can get stained with eosin as well as a multilobate nucleus.
  • Lymphocytes play an important role in antigen-oriented immune functions, involving either direct cytotoxicity (T cell) or antibodies (B cell). They have a big nucleus and extremely little cytoplasm. One cannot differentiate between T and B cells via light microscopy.
  • The other cells are WBC precursors.

The use of peripheral blood smears in detection of Malaria

The most consistent and popular method of diagnosing malaria is microscopic investigation of peripheral blood smears. This is because every one of the four main parasite species causing malaria has distinct characteristics. Two types of peripheral blood smears are generally used for this purpose. The thin peripheral blood smears are alike the normal blood films and permit identification of species, due to the better preservation of the parasites features in this preparation.

On the other hand thick peripheral blood smears allow the lab technician to examine a larger quantity of blood and are nearly 11 times more responsive as compared to the thin peripheral blood smears. Therefore, thick blood films allow for easy detection of low levels of infection. However, the characteristics of the parasite are quite distorted as compared to the thin blood films, and hence differentiating between the various species is a lot more problematic.

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