Blood Smear
How is it used?
A peripheral blood smear was once prepared on nearly everyone who had a complete blood count (CBC) performed. With the automated blood cell counting instruments currently used, an automated differential is also provided. However, if the presence of abnormal WBCs, RBCs, or platelets is suspected, a blood smear examined by a trained eye is still the best method for definitively evaluating and identifying immature and abnormal cells.
There are many diseases, disorders, and deficiencies that can have an effect on the number and type of blood cells produced, their function, and their lifespan. Although usually only normal mature cells are released into the bloodstream, circumstances can force the bone marrow to release immature and/or malformed cells into the circulation. When a significant number of abnormal cells are present, they can suggest an underlying condition and prompt the doctor to do further testing.
When is it ordered?
The blood smear is primarily ordered to evaluate blood cell populations when a CBC with differential, performed with an automated blood cell counter, indicates the presence of abnormal or immature cells. It may also be performed when a doctor suspects a deficiency, disease, or disorder that is affecting blood cell production, such as an anemia, decreased or abnormal production of cells in the bone marrow, or increased cell destruction. A blood smear may also be ordered when a patient is being treated or monitored for a blood cell-related disease.
What does the test result mean?
Findings from the blood smear evaluation are not always diagnostic in themselves and more often indicate the presence of an underlying condition and its severity and suggest the need for further diagnostic testing. Blood smear findings may include:
Normal, mature red blood cells are uniform in size (7 µm) 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 with routine staining. While not every RBC will be perfect, any significant number of cells that are different in shape or size may indicate a more severe problem. There may be one or more irregularities present and may include:
Anisocytosis - variable sizes of red cells. The presence of smaller RBCs (<7µ m is referred to as microcytosis and RBCs larger than 7 µ m is macrocytosis. Poikilocytosis - various shapes of red cells. These may include echinocytes, acanthocytes, elliptocytes, keratocytes, rouleaux, sickle cells, target cells, teardrop cells, and shistocytes.
White blood cells have a nucleus surrounded by cytoplasm. All WBCs are derived from bone marrow stem cells. In the marrow, they differentiate into two groups: myelocytic and lymphoid cells. They then mature into five distinct types of WBCs. - Neutrophils - cells that have cytoplasm with pink or purple granules. They compose the majority of WBCs in a healthy adult.
- Eosinophils - are easily recognized in stained smears with their large, red-orange granules. Generally low in number (1-3%), most often become elevated in number in individuals with allergies and parasitic infections.
- Basophils - all have large, black granules and are the least often seen type of WBC (1%). Increased numbers of basophiles are not often encountered but may be elevated in certain leukemias, chicken pox, ulcerative colitis, or after an immunization.
- Monocytes - are usually the largest of the WBCs (12-20 µm) and are often referred to as scavenger cells (phagocytes). They can ingest particles such as cellular debris, bacteria, or other insoluble particles.
- Lymphoid cells - lymphocytes are smaller in size (10-12 µm) and have a homogeneous cytoplasm and a smooth, round nucleus. These cells are responsible for the production of antibodies (immunoglobulins).
Platelets
These are cell fragments that derive 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. You must have a sufficient number of platelets to control bleeding. If there are too few, 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.
Enumeration of platelets is usually part of a CBC. An abnormally low number or high number of platelets may be further evaluated by preparing a peripheral blood smear to directly visualize any anomalies in shape or size.
