Test Quick Guide

A blood smear is sometimes prepared when red blood cell and/or white blood cell abnormalities are suspected when an automated Complete Blood Count (CBC) is performed. Visual examination of a blood smear provides a clearer picture of these abnormalities prompting further testing to determine the cause.

A blood sample is drawn from a vein in your arm or by pricking a finger (or using a heel stick if an infant), then spread thinly onto a glass slide. The slide is then treated with a special stain to characterize the white blood cells, the red blood cells, and the platelets. Using a microscope, the blood cells on the slide are examined and evaluated by a trained clinical laboratory scientist (CLS) or a pathologist. Automated digital systems are used by most laboratories to help analyze blood smears more effectively and efficiently.

About the Test

Purpose of the test

A blood smear is used to evaluate your red blood cells (RBCs), noting any abnormal differences in size, shape, or other physical appearances such as that seen in various anemias, sickle cell disease, Thalassemia, or other disorders.

Evaluation of white blood cells (WBCs) is required especially if they are increased or decreased in number and noting the types of WBCs that are present.  Enumerating platelets is important as they play a major role in the clotting process. Too few platelets can result in abnormal bleeding and too many can cause life-threatening clots.

Examining a blood smear allows for the confirmation that platelet numbers are adequate and that the size and shape of the platelets are appropriate. Any WBC, RBC, or platelet abnormalities observed on the blood smear provides important information in determining a diagnosis, treatment, and management of a patient.

What does a blood smear measure?

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) — WBCs are produced in the bone marrow and released into your bloodstream to help fight infections or participate in immune responses.
  • Red blood cells (RBCs, erythrocytes) — carry oxygen from the lungs to your tissues and return to the lungs carrying carbon dioxide.
  • Platelets (thrombocytes) — are small cell fragments that are vital to proper blood clotting.

These cell populations are produced in the bone marrow and are eventually released into the bloodstream as needed. The number and type of each cell present in the blood are dynamic but are 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). The five types of WBCs are neutrophils, lymphocytes, monocytes, eosinophils, and basophils. The CBC evaluates the number of each type of cell present, looking for any abnormal changes.
  • 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 support findings from other tests. For example, RBCs that appear smaller and paler than normal may support other results that indicate a type of anemia. Similarly, the presence of not fully mature WBCs may add to the information from other tests to help make a diagnosis of infection, malignancy, or other conditions.

When should I get a blood smear?

You should get a blood smear if your complete blood count (CBC) and/or automated WBC differential results are abnormal. Also, take this test when you have signs and symptoms that a health care practitioner suspects are due to a condition affecting your blood cells. These symptoms may include:

  • Bone pain
  • Fatigue or weakness
  • Fever or chills
  • Jaundice (yellowing of the skin and eyes)
  • Pale skin
  • Unusual bleeding (including nose bleeds)
  • Losing weight
  • Swollen glands (lymph nodes)

A blood smear may also be recommended by your doctor if you have been exposed to ticks, though suggestive, this is not relied upon as a diagnosis. If you have recently traveled to a developing country, certain parasites, such as those seen in malaria, attack RBCs and can be observed under the microscope.

Finding a Blood Smear Test

How can I get a blood smear?

A blood smear is typically 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.

A blood smear may also be ordered on a regular basis if you are being treated or monitored for a blood cell-related disease.

Can I take the test at home?

Due to the nature of the test and sample collection, a blood smear cannot be done at home and must be administered by a health professional.

How much does the test cost?

Several factors can affect the cost of a blood smear, including the cost of the phlebotomy, fees for analysis, and charges for office visits. These costs are often covered by health insurance if the test is prescribed by your doctor.

Check with your doctor and insurance provider to find out about copays, deductibles, and any other charges if you get a blood smear.

Taking a Blood Smear Test

Blood smears require a blood sample to be taken, typically drawn from a vein in your arm or by pricking a finger. The test is administered in a clinical setting by a health care professional.

Before the test

There is no preparation required prior to having a blood smear test.  However, if your doctor has ordered other blood tests to be run in conjunction with the blood smear, you may be required to fast for several hours prior to the test.

During the test

A medical professional will take a blood sample from a vein in your arm using a needle.  You will have an elastic band tied around your upper arm to make the veins in your arm stand out. The inside of your arm will be cleaned with an antiseptic pad and allowed to dry. A needle will be inserted into the vein, a vial of blood will be drawn, and then the needle will be removed. The entire process usually lasts less than a few minutes during which there may be some temporary pain as the needle is inserted and withdrawn.

After the test

After blood is drawn with a needle, a bandage is applied, and you may need to apply light pressure for a few minutes to help prevent bleeding. There may be tenderness at the puncture site, and some light bruising may occur. You can usually return to normal activities once the test is complete.

Blood Smear Test Results

Receiving test results

It can take anywhere from a few hours to a few days to receive your blood smear test results, depending on your proximity to the laboratory where your sample is examined.  Your healthcare provider who ordered the test will discuss the results with you and a test report can be sent to you either electronically or by mail.

Interpreting test results

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.  A blood smear is often used to categorize and/or identify conditions that affect one or more types of blood cells and to monitor individuals undergoing treatment for these conditions.

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 abnormal number or type of cells is present, it can suggest a disease or condition that will prompt a health care practitioner to do further testing.

There are many diseases, disorders, and deficiencies that can affect the number and type of blood cells produced, their function, and their life span. Examples include:

  • Anemia, such as iron deficiency anemia, hemolytic anemia, aplastic anemia, sickle cell anemia
  • Myeloproliferative neoplasms are cancers of certain WBC types (myeloid cells) and platelets often resulting in a type of leukemia
  • Lymphocytic leukemia
  • Other bone marrow disorders such as multiple myeloma, Waldenstrom’s macroglobulinemia, hairy cell leukemia, aplastic anemia, and polycythemia vera

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 your clinical signs and symptoms.

You may want to ask your doctor follow-up questions after you receive your results, such as:

  • Are any follow-up tests required?
  • What possible diagnoses do the results indicate?
  • Are there side effects or downsides to treatment?
  • Are there any changes I should make in my lifestyle as a part of the treatment?
  • How will I know if the treatment is working?

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