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
A blood sample drawn from a vein in your arm or by pricking a finger or, in the case of an infant, a heelstick
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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:
- White blood cells (WBCs, leukocytes) — help fight infections or participate in immune responses
- Red blood cells (RBCs, erythrocytes) — carry oxygen to tissues
- Platelets (thrombocytes) — small cell fragments that are vital to proper blood clotting
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:
- Compares the WBCs' size, shape, and general appearance to the established appearance of "normal" cells. It also determines the five different types of WBCs and their relative percentages (manual WBC differential).
- Evaluates the size, shape, and color (indicators of hemoglobin content) of the RBCs (RBC morphology)
- Estimates the number of platelets present
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.
How is the blood smear used?
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.
When is it ordered?
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:
- Weakness, fatigue
- Pale complexion
- Unexplained jaundice
- Excessive bleeding episodes, easy bruising, or frequent nose bleeds
- Enlargement of the spleen
- Bone pain
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.
What does the test result mean?
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:
- Hemoglobin variants including sickle cell anemia and thalassemia
- Myeloproliferative or myelodysplastic neoplasms
- Bone marrow disorders
There may be one or more RBC irregularities seen on a blood smear. Two examples include:
- Anisocytosis — variable sizes of red blood cells may indicate anemia; RBCs smaller than normal are referred to as microcytes and RBCs larger than normal are called macrocytes.
- Poikilocytosis — various shapes of red cells; these may include burr cells (echinocytes), acanthocytes, elliptocytes, rouleaux, sickle cells, target cells, teardrop cells, and schistocytes (red cell fragments, helmet cells).
- Anisopoikilocytosis — variability in both RBC size and shape
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:
- Neutrophils (10-18 µm) are cells that have cytoplasm with pink or purple granules. They comprise the majority of WBCs in a healthy adult. They are involved in the defense against infections.
- Eosinophils (10-15 µm) are easily recognized in stained smears with their large, red-orange granules. Generally low in number (1-3%), they most often increase in number in individuals with allergies and parasitic infections.
- Basophils (10-15 µm) have large, purple-black granules and are the least often seen type of WBC (1%).
The non-granulocytes include:
- Monocytes (which are also of myeloid lineage) 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.
- Lymphocytes are often smaller in size (10-12 µm) and usually have a small amount of cytoplasm and often a smooth, round nucleus although some are larger and can contain a greater amount of cytoplasm. They usually represent the most common type of WBC in younger children. One type of lymphocyte, the B-cell, is responsible for the production of antibodies (immunoglobulins).
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:
- Infections and/or inflammation — can increase certain types of WBCs
- Bone marrow disorders — depending on the condition, may increase or decrease absolute and relative numbers of WBCs
- Allergies — may affect the number of eosinophils
- Leukemia, myelodysplastic syndrome or myeloproliferative neoplasm — immature white blood cells such as blasts may be seen on the blood smear; blasts are normally found in the bone marrow where WBCs are produced and mature before being released into the blood. If blasts are seen on a blood smear, they may indicate a serious bone marrow disease. They can also be seen in other scenarios, such as when the bone marrow is recovering or regenerating from chemotherapy, or, as another example, stimulated by a medication prior to stem cell collection.
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.)
Is there anything else I should know?
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).
If my blood smear shows abnormal results, what additional testing might be done?
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:
Why hasn't the automated blood cell counter totally replaced the blood smear?
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.
If a drop of blood on a slide contains millions of RBCs, thousands of WBCs, and hundreds of thousands of platelets, how can the laboratorian see individual cells?
- Anisocytosis: this is a variation in size of RBCs; it may be an indication of anemia
- Macrocytosis: large RBCs that may be due to a vitamin B12 or folate deficiency. They are seen in megaloblastic anemia (e.g., pernicious anemia), chronic alcoholism, liver disease, thyroid disease, and myelodysplastic syndrome.
- Microcytosis: this is the presence of small RBCs that is commonly due to an iron deficiency anemia or to an inherited disorder such as thalassemia.
- Poikilocytosis is a variation in the shape of RBCs and may include several different abnormalities at the same time.
- Acanthocytes (spur or spiculated cells): irregular shaped cells with 5-10 spicules; may be present in the blood of people who have had their spleen removed (splenectomy) or with liver disease. They are also present in an inherited disorder called abetalipoproteineimia.
- Echinocytes (burr or crenated): may have 10-30 blunt projections and often are seen in people with renal failure; may be an artifact – something caused during sample preparation.
- Elliptocytes (ovalocyte): elliptical-shaped RBCs seen in hereditary elliptocytosis and various anemias, myelofibrosis.
- Fragmented cells (schistocytes, helmet cells): fragmented RBCs of various shapes that may be seen in people with disseminated intravascular coagulation (DIC), hemolytic uremic syndrome, thrombotic thrombocytopenic purpura, or a vascular artificial device (prosthesis) such as a mechanical heart valve.
- Rouleaux: RBCs that appear as a stack of coins and are seen in people with multiple myeloma or Waldenstrom macroglobulinemia because of increased serum proteins.
- Sickle cells: crescent-shaped RBCs that are characteristic of sickle cell anemia.
- Target cells: RBCs that resemble a bull's-eye; commonly seen in people with abnormal inherited forms of hemoglobin (hemoglobinopathies), thalassemia, various anemias and liver disease.
- Teardrop cells (dacrocytes): RBCs that resemble a teardrop; often seen in people with myelofibrosis and thalassemia.
- Spherocytes: sphere-shaped RBCs that are often present in immune hemolytic anemia or hereditary spherocytosis.
- Hypochromia/hypochromasia: this may be seen in a variety of disorders, including thalassemia and iron deficiency. The RBC is pale in color due to insufficient hemoglobin and contains a large, hollow middle (central pallor) of the cell.
- Hyperchromia/hyperchromasia: the RBC is darker in color than normal due to a greater than normal concentration of hemoglobin within the cell; this may be due to dehydration or presence of spherocytes.
- Polychromasia: blue-staining RBCs reflecting increased RNA content, indicating that they are immature due to early release from the bone marrow.
Abnormal structures within the red blood cell
- Nucleated RBCs (NRBC, normoblasts): an immature form of RBCs seen when there is increased demand for RBCs to be released by the bone marrow, or when there is marrow involvement by fibrosis or tumor; may be seen in severe anemia, myelofibrosis, thalassemia, miliary tuberculosis, cancers that involve the bone marrow, and in chronic low oxygen levels (hypoxemia). Nucleated RBCs can be normal in infants for the first two or three days after birth.
- Reticulocytes: these are immature RBCs that are usually a blue-staining (polychromatic) color. A few of these young red blood cells are normal in the circulation, but they may be present in elevated numbers with acute blood loss, hypoxia, RBC destruction, sickle cell disease, glucose-6-phosphate. dehydrogenase (G6PD) deficiency, and autoimmune hemolytic anemia.
- Siderocyte, sideroblast, ring sideroblast: When RBCs are stained with Prussian blue dye, iron granules may be seen. Sideroblasts are immature siderocytes and may actually form a ring pattern indicative of sideroblastic anemia.
- Basophilic stippling (punctate basophilia) is dark blue dots inside the RBC; due to abnormal aggregation of ribosomes and polyribosomes and may be present in heavy metal intoxication (such as lead), thalassemia and many other anemias.
- Heinz bodies: large inclusion bodies (granules) in the RBCs when stained with a supravital stain like crystal violet; often seen in the setting of an enzyme (G6PD) deficiency, drug-induced hemolysis, or unstable hemoglobin disease.
- Howell-Jolly bodies (small, round remnants of nuclear DNA inside cell): present in sickle cell anemia, hemolytic or megaloblastic anemias, and may be seen after a splenectomy.
- Cabot ring: uncommon threadlike inclusions that form a loop or figure-of-eight structure within the RBC; may be seen in a variety of anemias.
- Malarial parasites: in people with malaria, these parasites live inside RBCs and may be visible on a blood smear. This is not a routine finding; these parasites are usually found in those who live in or have traveled to areas where the disease is endemic.
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:
- Toxic granulation: large dark blue granules in the cytoplasm, associated with severe infection, burns, trauma, and G-CSF colony stimulating factor therapy; often reflects accelerated neutrophil maturation.
- Vacuolization: vacuoles appear as holes in the cytoplasm and are frequently found in association with toxic granulation but can also be a degenerative feature seen when the blood is stored for a prolonged period of time before preparing the blood smear.
- Döhle bodies: irregular grayish or bluish inclusions in the peripheral cytoplasm of neutrophils; they are denatured aggregates of free ribosomes or rough endoplasmic reticulum that are often seen in association with toxic granules and vacuoles. They may be present in association with infections, burns, trauma, or with exposure to cytotoxic agents (i.e., chemotherapy). They may also be seen after cytokine stimulation (e.g., G-CSF) or during a normal pregnancy.
- Auer rods: unique, pink or red rod-shaped inclusions that are seen in very immature myeloid cells ("blasts") or rarely more mature neutrophils in people with acute myeloid leukemia or high-grade myelodysplastic syndrome.
- Bands - increased numbers: slightly immature neutrophils are normal in the circulation in small numbers, but if there is a percentage increase of them, there is said to be a "left shift." This may happen when an acute infection stimulates increased neutrophil production, causing the bone marrow to prematurely release some WBCs before they have matured to the neutrophil stage. Other immature forms that may sometimes be seen on a blood smear include myelocyte and metamyelocyte or even promyelocyte and myeloblast.
- Hypersegmentation: neutrophils with six or more nuclear segments; this is mainly associated with vitamin B12 and folate deficiency and myelodysplasia but can also be seen with alcoholism and rarely as a hereditary condition.
- Pelger-Huët anomaly: hereditary anomaly where neutrophils appear with fewer than two lobes; the nucleus is often in the shape of a peanut or dumbbell, or may consist of two lobes connected with an obvious filament. They may also appear with certain immunosuppressant drugs or in certain disease states such as myelodysplasia and is referred to a "pseudo- Pelger-Huët anomaly".
- Alder-Reilly granules: large, dark leukocyte granules that stain purple; they suggest mucopolysaccharidosis (an inherited enzyme deficiency disorder, examples being Hurler's and Hunter's syndromes).
- Chédiak-Higashi granules: an inherited anomaly characterized by the presence of big red, blue, or greenish granules that may be found in granulocytes, lymphocytes, and monocytes. People with this syndrome may have oculocutaneous albinism as well as a compromised immune system and photophobia.
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.
- Reactive lymphocyte (atypical lymphocyte, activated lymphocyte): these cells are large lymphocytes that contain a greater amount of cytoplasm and can vary in size and shape. Often a characteristic bluish tinge of cytoplasm is seen where the cell abuts with surrounding RBCs. Increased numbers of atypical lymphocytes are found in viral illnesses such as infectious mononucleosis.
- Hairy cells: these lymphocytes have tiny projections that make them appear hairy under the microscope; they are found in hairy cell leukemia.
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.