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To evaluate your red blood cells (RBCs), white blood cells (WBCs), and platelets, to distinguish between the different types of WBCs, and to determine their relative percentages in the blood; to help detect, diagnose, and/or monitor a range of deficiencies, diseases, and disorders involving blood cell production, function, and lifespan
When complete blood count (CBC) and/or automated WBC differential results are abnormal or when you have signs and symptoms that a healthcare practitioner suspects are due to a condition affecting your blood cells
A blood sample drawn from a vein in your arm or by pricking a finger or, in the case of an infant, a heelstick
A blood smear is a drop of blood spread thinly onto a glass slide that is then treated with a special stain and the blood cells on the slide are examined and evaluated. Traditionally, trained laboratorians have examined blood smears manually using a microscope. More recently, automated digital systems have become available to help analyze blood smears more efficiently.
A blood smear is a snapshot of the cells that are present in the blood at the time the sample is obtained. The blood smear allows for the evaluation of these cells:
These cell populations are produced and mainly mature in the bone marrow and are eventually released into the bloodstream as needed. The number and type of each cell present in the blood is dynamic but is generally maintained by the body within specific ranges.
The drop of blood on the slide used for a blood smear contains millions of RBCs, thousands of WBCs, and hundreds of thousands of platelets. A blood smear examination:
A variety of diseases and conditions can affect the number and appearance of blood cells. Examination of the blood smear can be used to support findings from other tests and examinations. For example, RBCs that appear smaller and paler than normal may support other results that indicate a type of anemia. Similarly, the presence of WBCs that are not fully mature may add to information from other tests to help make a diagnosis of infection, malignancy, or other conditions.
A blood smear is often used as a follow-up test to abnormal results on a complete blood count (CBC) to evaluate the different types of blood cells. It may be used to help diagnose and/or monitor numerous conditions that affect blood cell populations.
At one time, a blood smear was prepared on nearly everyone who had a CBC. With the development of more sophisticated, automated blood cell counting instruments, it has now become routine to provide an automated differential. However, if the results from an automated cell count and/or differential indicate the presence of immature, atypical, or abnormal white blood cells (WBCs), red blood cells (RBCs), and/or platelets or if there is reason to suspect that abnormal cells are present, then a blood smear may be performed.
A blood smear is often used to categorize and/or identify conditions that affect one or more type of blood cells and to monitor individuals undergoing treatment for these conditions. There are many diseases, disorders, and deficiencies that can affect the number and type of blood cells produced, their function, and their lifespan. Examples include anemia, myeloproliferative neoplasms, bone marrow disorders, and leukemia.
Usually, only normal, mature or nearly mature cells are released into the bloodstream, but certain circumstances can induce the bone marrow to release immature and/or abnormal cells into the circulation. When a significant number or type of abnormal cells are present, it can suggest a disease or condition and prompt a healthcare practitioner to do further testing.
The blood smear is primarily ordered as a follow-up test when a CBC with differential, performed with an automated blood cell counter, indicates the presence of atypical, abnormal, or immature cells. It may also be performed when a person has signs and symptoms that suggest a condition affecting blood cell production or lifespan.
Examples of signs and symptoms that may indicate one of these blood disorders include:
A blood smear may also be ordered on a regular basis when a person is being treated or monitored for a blood cell-related disease.
Findings from a blood smear evaluation are not always diagnostic in themselves and more often indicate the possibility or presence of an underlying condition, its severity, and the need for further diagnostic testing. The results are taken into consideration with the results of the CBC and other laboratory tests as well as the tested person's clinical signs and symptoms.
The results of a blood smear typically include a description of the appearance of the red blood cells, white blood cells, and platelets as well as any abnormalities that may be seen on the slide.
Red Blood Cells (RBCs)
Normal, mature red blood cells are uniform in size (7-8 µm in diameter) and do not have a nucleus as most other cells do. They are round and flattened like a donut with a depression in the middle instead of a hole (biconcave). Due to the hemoglobin inside the RBCs, they appear pink to red in color with a pale center after staining the blood smear. When the appearance of RBCs (RBC morphology) is normal, it is often reported as normochromic (normal color) and normocytic (normal size).
While not every RBC will be perfect, any significant number of cells that are different in shape or size may indicate the presence of disease. Some examples of conditions that can affect red blood cells include:
There may be one or more RBC irregularities seen on a blood smear. Two examples include:
See the section below for Details on Red Blood Cell Irregularities.
White Blood Cells (WBCs)
As part of a blood smear evaluation, a manual WBC differential may be performed. Typically, at least 100 WBCs are evaluated and categorized according to type. The percentage of each type is calculated. In addition, the appearance (morphology) and stage of development of the WBCs are noted. White blood cells have a nucleus surrounded by cytoplasm. All WBCs are derived from bone marrow stem cells. In the bone marrow, they differentiate into two groups: myeloid and lymphoid cells. They mature into five distinct types of WBCs.
Those with granules in their cytoplasm are also called granulocytes (they are of myeloid lineage) and include:
The non-granulocytes include:
Numerous diseases and conditions can affect the absolute or relative number of WBCs and their appearance on a blood smear. Examples of some of the conditions include:
See the section below for more Details on White Blood Cells.
These are cell fragments that develop from large bone marrow cells called megakaryocytes. Upon release from the bone marrow, they appear as fragments in the peripheral blood. When there is blood vessel injury or other bleeding, the platelets become activated and begin to clump together to form aggregates, which is the beginning of a blood clot.
There must be a sufficient number of platelets to control bleeding. If there are too few, or if they don't function properly, the ability to form a clot becomes impaired and can be a life-threatening situation. In some people, too many platelets may be produced, which may result in interferences with the flow of blood, increasing a person's risk of developing a blood clot. These same people may also experience bleeding because many of the extra platelets may be dysfunctional even though they appear normal.
A platelet count is usually part of a CBC. An abnormally low number or high number of platelets may be further evaluated by preparing a blood smear to directly visualize any anomalies in shape or size. For example, large platelets or giant platelets may be seen as the bone marrow tries to compensate for a low platelet count, but they can also be seen in myeloproliferative neoplasms, or immune thrombocytopenia, a condition in which the immune system inappropriately produces antibodies directed against platelets. (For more information, see the articles on Platelet Disorders and Excessive Clotting Disorders.)
Blood counts and morphology can also be affected during times of illness or stress, and after transfusion.
Findings on a blood smear that are abnormal are typically referred to a pathologist, often one with extensive experience in the study of blood (hematology), for further review and interpretation. Depending on the results, follow-up testing involving other bloodwork or even examination of a bone marrow aspirate and biopsy may be required for a diagnosis.
Although uncommonly used for this purpose in the U.S., blood smears may be used to help diagnose malaria, a disease caused by a blood parasite. The parasite may be seen when a blood smear is examined under a microscope. Malaria is usually only seen in travelers returning from areas where the parasite is more common (endemic).
Depending on the findings from the CBC and blood smear and the condition that your healthcare practitioner suspects, follow-up testing may include tests such as:
It has on a routine basis, but the automated blood cell counter usually evaluates the red blood cells (RBCs), white blood cells (WBCs), and platelets based on their shape, size, and electrical or photometric properties. There can be some variation in each cell type and numbers the body produces. Use of an automated instrument can often identify the presence of abnormal cells but lacks the ability to definitively subclassify them. Cell fragments and platelet clumps, particularly if they are large in size, can be mistakenly counted as WBCs, thus falsely elevating a white cell count. A laboratorian can see these abnormalities on a blood smear and has been trained to identify and classify them appropriately.
Traditionally, blood smear review/interpretation has been done manually using a microscope, which is time-consuming. Automated digital morphology systems are now available to help analyze blood smears more efficiently.
A specific technique is used to spread the drop of blood out across a glass slide into a "thumbprint" shape. Along the edges of this thumbprint the cells are only a single layer thick. This allows the cells in this area to be individually counted and evaluated under the microscope.
NEUTROPHILS (also called segmented neutrophils, segs, polymorphonuclear cells, polys, or PMNs) are about 12 microns in diameter and their function is to engulf and destroy invading organisms. They normally make up about 50-70% of the total WBC count in the blood of older children and adults and may have two to five nuclear lobes connected by a thin strand of nuclear material. This type of WBC may be seen in greater numbers during infections, malignancies, or extreme stress situations. The cytoplasm of neutrophils is pale and often contains small pink to purple granules. These granules (specific granules and azurophilic granules) contain certain enzymes and proteins that neutralize or destroy microbes. Bands are immature neutrophils with a U-shaped nucleus.
Anomalies of neutrophils may include:
Lymphocytes are relatively small (7-10 µm) and round in shape. The nucleus is generally large in relation to the amount of cytoplasm. The cytoplasm is pale blue and normally only a small proportion of lymphocytes has any granules. The nucleus of most lymphocytes is smooth in appearance and is dark blue.
There are two major types of lymphocytes, B cell and T cell, but they cannot be distinguished when viewed under the microscope using standard staining techniques. B cells can be differentiated from T cells using specific fluorescent-labeled antibody stains in conjunction with a special instrument called a flow cytometer. B cells create specific antibodies while T cells can activate B cells as well as recognize and destroy invading organisms. Lymphocytes normally make up about 20% to 40% of the total WBC count in adults and a higher proportion in infants and young children.
Eosinophils have two or three lobes to their nucleus and contain characteristic reddish/orange granules in their cytoplasm. They are most often involved in allergic responses and parasitic infections. Normally only about 1-4% of WBCs in the blood are eosinophils.
Monocytes are the largest in size of the WBCs and comprise less than 6% in normal blood. They are characterized by their abundant blue-grey cytoplasm that is irregular in shape and have a folded nucleus. The main function of monocytes is to ingest microbes and respond to infection and inflammation by releasing certain proteins (monokines) that can inactivate bacteria. When stimulated by cytokines, monocytes can move out of the blood and become tissue macrophages.
Basophils have a multi-lobed nucleus and have many dark blue granules (which contain histamines) in their cytoplasm. Only about 1% of WBCs are basophils. A sight elevation in number may be seen during an allergic response, ulcerative colitis, chronic sinusitis, chickenpox, or after immunizations. A significant increase is common in chronic myeloid leukemia.
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