What Is a Blood Smear?

What to Expect When Undergoing This Test

A blood smear, also referred to as a peripheral smear for morphology, is an important test for evaluating blood-related problems, such as those in red blood cells, white blood cells, or platelets. It has a wide range of uses, including distinguishing viral infections from bacterial infections, evaluating anemia, looking for causes of jaundice, and even diagnosing malaria.

Unlike automated tests (such as a CBC), a technician or physician looks at a blood smear under the microscope in order to detect a wide range of changes that give clues to underlying diseases.

Purpose of Test

A blood smear involves looking at a sample of blood under the microscope after applying special stains and looking for abnormalities or changes in red blood cells, white blood cells, and platelets.

Uses

There are many reasons why your doctor may order a blood smear. Some of these include:

  • to further evaluate abnormalities found on a complete blood count (CBC) such as a high or low red blood cell count, white blood cell count, or platelet count.
  • to evaluate an infection (identifying the types of white blood cells present can help determine if an infection is viral, bacterial, or parasitic, as well as the severity)
  • to look for causes of unexplained jaundice
  • as part of a work-up for people who have unexplained weight loss (defined as a loss of 5 percent of body weight over a 6 month period without trying)
  • to evaluate symptoms of lightheadedness and palor (paleness)
  • to look for causes of petechiae, bruising, or excess bleeding
  • with a low platelet count, to determine if the cause is increased degradation or decreased production (based on the size)
  • to investigate findings suspicious for blood-related cancers
  • to look for malaria
  • to confirm sickle cell disease
  • to evaluate symptoms of bone pain
  • to look for causes of enlargement of the spleen, liver, or lymph nodes.

What a Blood Smear Measures

A blood smear looks for the numbers and characteristics of the three types of blood cells:

  • Red blood cells (RBCs) are the cells that transport oxygen to the tissues
  • White blood cells (WBCs) are cells that fight infection among several other functions
  • Platelets are cell fragments that play an important role in blood clotting

Findings that are noted include:

  • The number of the type of blood cells
  • With white blood cells, the number and proportion of the different subtypes of white blood cells, including lymphocytes, neutrophils, basophils, eosinophils, and monocytes
  • The relative size of the cells, as well as a variation in size
  • The shape of the blood cells
  • Other characteristics such as inclusions in the blood cells, clumping of cells, or cell fragments other than platelets
  • Other findings in the blood such as the presence of malaria parasites

Meaning

In addition to information about the different types of blood cells, a blood smear (especially when combined with a reticulocyte count) can often be a good measure of how well the bone marrow is functioning.

Limitations

There are a few limitations to a blood smear. If a person has received a blood transfusion, the smear will include a combination of native and donated blood cells.

There are several potential ways in which error can enter into a blood smear. Delays in making the slide after blood is drawn, exposure to extreme temperatures, or clotting can result in a poor sample. Preparation of the slide requires careful technique, and results may be inaccurate if the slide is too thin or too thick.

Since the measurements are made by a person rather than a machine, the experience of the person analyzing the smear (sometimes called a hematopathologist) can affect the interpretation.

Similar Tests

A blood smear is often done in conjunction with a CBC and indices, and it is the combination of these studies that is most helpful. A blood smear also gives a "second read" to results obtained on the CBC.

A complete blood count (CBC) provides the number of:

  • Red blood cells
  • White blood cells
  • Platelets

Red blood cell and platelet indices further describe the cells that are present and include

  • Mean corpuscular volume (MCV): A measure of the size of red blood cells
  • Mean corpuscular hemoglobin concentration (MCHC): A measure of the hemoglobin concentration of the cells
  • Red cell distribution width (RDW): A measure of the variation in size of red blood cells
  • Mean platelet volume (MPV): A measure of the size of the platelets

Complementary Tests

Other tests that may be done in conjunction with a blood smear include:

  • Reticulocyte count: A reticulocyte count is a measure of the number of immature red blood cells and is a good measure of how well the bone marrow is functioning
  • Bone marrow aspiration and biopsy
  • Chemistry panel (including kidney and liver function tests)
  • Thyroid tests

Risks and Contraindications

There are few risks associated with a blood smear, though bleeding could be of more concern for those who have a low platelet count or are on blood thinners.

The Test

A blood smear can be drawn in the hospital and in most clinic settings. Some clinics have a lab on site that performs the test, while others send the sample out to a reference lab.

Before the Test

There is no special preparation prior to having a blood smear drawn, and no eating or activity restrictions are needed. It's important to bring your insurance card to your appointment as well as any copies of medical records you were asked to collect.

With a blood smear, it's helpful for doctors to have any previous blood tests you have done with which to compare current findings.

During the Test

The technician will begin by locating a vein and then cleansing the area with antiseptic. A tourniquet is applied to make the vein engorged and more visible, and then the technician will insert the needle into your vein. There may be a sharp sting when the needle is inserted and some slight pressure as the sample is taken.

When the needle is removed, you will be asked to hold pressure over the site of the blood draw to limit bleeding and then a bandage will be applied.

After the Test

You will be able to leave the lab when your test is done and either return to your room to visit with your doctor, or return home and receive a call with the results. Side effects are uncommon but may include bruising at the site of the blood draw, persistent bleeding, and rarely, infection.

Slide Preparation

When your sample arrives in the lab, a technologist will carefully prepare the slide. This involves placing a drop of blood on the slide and then carefully spreading the blood along the slide so that on a sample of 200 blood cells, there is space between the cells.

Interpreting Results

The blood smear can reveal important information about many blood-related conditions as well as kidney disease, liver disease, and more. Sometimes a diagnosis can be made based on the blood smear alone (such as with hereditary elliptocytosis) and other times further testing will be indicated.

Reference Range

Reference ranges exist for the number of red blood cells, white blood cells, and platelets, as well as the expected appearance of these cells. Each of the types of blood cells is evaluated for number, shape, size, and other characteristics.

Red Blood Cells

The blood smear takes a second look at several findings from a CBC including:

  • Red blood cell count.
  • Anisocytosis, or the variation in sizes of the red blood cells, and whether the cells are larger than normal, normal, or smaller than normal. Small red blood cells are referred to as microcytes and large red blood cells are referred to as macrocytes. This is correlated with the MCV and RDW. Large cells are often seen with vitamin B12 and folate deficiency, and small cells are often seen with iron deficiency anemia and thalassemia.
  • The degree of color. Dark red cells are considered hyperchromic and light red cells are referred to as hypochromic. This correlates with the MCHC. Light colored cells are often seen with iron deficiency anemia.
  • Poikilocytosis, or the shapes of the red blood cells. This can include shapes such as teardrop cells (shaped like a teardrop), spherocytes, and more as discussed below.
  • Anisopoikilocytosis, or the variation in both size and shape of the red blood cells.
  • Presence of inclusions, including parasites
  • Abnormal red blood cells. Only one percent or less of the red blood cells should have nuclei (nucleated RBCs).

    Findings

    There are several other findings that may be seen on examination of the RBCs and is one of the great advantages of the blood smear over automated tests. Some of these include:

    • Burr cells (echinocytes), which are seen with kidney failure
    • Target cells, which are seen with abnormal hemoglobins
    • Acanthocytes or spur cells (RBCs with thorny projections), seen with alcoholic cirrhosis and other conditions
    • Elliptocytes, seen with hereditary elliptocytosis
    • Spherocytes, seen with hereditary spherocytosis and extravascular hemolysis
    • Sickle cells, seen with sickle cell disease
    • Teardrop cells (dacrocytosis), seen with bone marrow fibrosis and myeloproliferative diseases
    • Shistocytes (red blood cell fragments), seen with hemolytic anemias
    • Helmet cells, seen with intravascular coagulation hemolysis
    • Basophilic stippling (ribosomes clumped together in the cells), seen with toxic injury to the bone marrow such as with lead poisoning
    • Rouleaux formation, which refers to stacks of RBCs that are stuck together and can be a sign of connective tissue diseases, diabetes, cancers such as multiple myeloma, or an allergic reaction to antibiotics. Rouleaux formation (and consequently, having the RBCs becoming stuck in capillaries) is an underlying process behind diabetic retinopathy.
    • Nucleated red blood cells, seen with severe hemolysis
    • Howell-Jolly bodies, seen in people who have had their spleen removed and in people with megaloblastic anemia (vitamin B12 and folate deficiency)
    • Heinz bodies or bite cells, seen when denatured hemoglobin is clumped in RBCs
    • Cabot's rings (parts of the nucleus that are left over), seen uncommonly in vitamin B12 deficiency (pernicious anemia) and lead poisoning
    • Parasites such as malaria parasites or Bartonella parasites may be seen within the RBCs

    White Blood Cell

    The white blood cell count on a blood smear gives important information about the number of the different types of blood cells as well as other findings. When a particular type of white blood cells is increased, it can give important clues about underlying problems.

    Types and Number of WBCs

    • Lymphocytes: Increases seen with viral infections and some leukemias.
    • Neutrophils: Increases seen with bacterial infections, trauma, and some leukemias.
    • Eosinophils: Increased numbers are often seen with allergies and asthma. Very high levels are often associated with parasitic infections.
    • Basophils: Increases can be a cause for concern and are often seen with cancer.
    • Monocytes: This type of white blood cell can be pictured as a garbage can and can be elevated with many conditions.

    Maturity of White Blood Cells

    The maturity of white blood cells can give information about the severity of infection or suggest an underlying leukemia.

    • Bands: These are young white blood cells and are often increased with serious infections. A band count greater than 15 percent is referred to as a "left shift."
    • Other immature white blood cells should not be seen on a blood smear (in peripheral blood) and raise suspicion about leukemia. This includes finding myeloblasts, metamyelocytes, promyelocytes, myelocytes, or with lymphocytes, finding lymphoblasts or prolymphocytes.

    Abnormal White Blood Cells

    Atypical lymphocytes (more than 5 percent) are often seen with infectious mononucleosis

    Other Findings

    In addition, other findings that may be noted include:

    • Toxic granulations (in neutrophils): Seen with severe infections
    • Hypersegmented neutrophils (more than 5 nuclear lobes): Seen with vitamin B12 and folate deficiency as well as myeloproliferative disorders
    • Bright green inclusions: Sometimes seen in liver failure and associated with a poor prognosis
    • Bilobed neutrophils: Seen in some genetic syndromes

    Platelet Number and Findings

    The number of platelets is important to note, especially if low (thrombocytopenia). The diagnosis can be narrowed down by determining if the platelets are smaller than normal or larger than normal. Other findings may include hypogranular platelets.

    Follow-Up

    Follow-up after a blood smear will depend on the findings of the test. In some cases, such as with hereditary elliptocytosis, the findings on the smear are enough to make the diagnosis. Findings such as immature white blood cells indicate a bone marrow study should be done.

    A Word From Verywell

    A blood smear is a fairly inexpensive test that can provide a large amount of information when it comes to evaluating many diseases. While automated tests are rapid and cost-effective in some settings, we don't have technology that can replace the human eye in distinguishing the many subtle changes in blood cells that give important clues in diagnosis.

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