Blood Smear: Uses, Side Effects, Procedure, Results

What to Expect When Undergoing This Test

A blood smear is a test for detecting problems in red blood cells, white blood cells, or platelets. It's sometimes called a peripheral smear for morphology. The test has a wide range of uses. It can be used to tell whether an infection is viral or bacterial. It can also detect anemia, find causes of jaundice, and diagnose malaria.

A blood smear is different from automated tests such as a complete blood count (CBC). A technician or healthcare provider typically looks at a blood smear under a microscope. The blood smear results and interpretation might give them clues to help diagnose a problem.

This article will review what a blood smear looks for, how it's done, and what the results mean.

preparing a blood smear for evaluation
Zaharia_Bogdan / iStock

Purpose of Test

A blood smear test involves looking at a sample of blood under a microscope. First, special stains are applied to the sample. They highlight the number, shape, and any abnormalities or changes in three types of cells evaluated in the blood smear. These blood cells are:

  • Red blood cells (RBCs), which transport oxygen to the tissues
  • White blood cells (WBCs), which fight infection
  • Platelets, the cell fragments that help blood clot

Reasons for a Blood Smear Test

Your healthcare provider may order a blood smear for many reasons. Some of these include:

  • To investigate high or low red blood cell count, white blood cell count, or platelet count on a CBC
  • To identify the types of white blood cells present, which can help determine if an infection is viral, bacterial, or parasitic
  • To look for causes of jaundice
  • To explain unintentional weight loss
  • To look for causes of petechiae, bruising, or excess bleeding
  • To find out if platelet count is low because cells are being destroyed, or if it's because the body is producing fewer of them
  • To detect 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 Can a Blood Smear Show?

Information found in the peripheral smear results could include:

  • The number of each type of the blood cells
  • The number and proportion of each type of white blood cell, including lymphocytes, neutrophils, basophils, eosinophils, and monocytes
  • The size of the cells, as well as differences in size
  • The shape of the blood cells
  • The presence of inclusion bodies, clumps, or fragments of material in the cells
  • Other findings in the blood such as the presence of malaria parasites

A blood smear is often a good measure of how well the bone marrow is functioning. When this is the aim, a blood smear test often includes a reticulocyte count. Reticulocytes are red blood cells that haven't fully developed yet.

It's important to know that if you have had a blood transfusion, the smear will include both your own blood cells and the cells of the donor.

Certain errors can affect how accurate the blood smear results are. These errors include:

  • A delay in making the slide after blood is drawn
  • Exposure to extreme temperatures
  • Clotting
  • Preparing a slide that's too thin or too thick

Blood smears are measured by a person rather than a machine. The experience of the person analyzing the smear (called a hematopathologist) can affect how it is interpreted.

Blood Smear Test and the CBC

A blood smear is often done along with a complete blood count (CBC) and indices. Looking at both studies together can yield more helpful results. A blood smear interpretation gives a "second read" to results on the CBC.

A CBC provides the number of red blood cells, white blood cells, and platelets. Red blood cell and platelet counts also describe:

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

  • Reticulocyte count
  • Bone marrow aspiration and biopsy
  • Chemistry panel, including kidney and liver function tests
  • Thyroid tests

There are few risks associated with the blood smear procedure. Bleeding may be a concern if you're taking blood thinners or you have a low platelet count.

A blood sample can be drawn in the hospital and in most clinic settings. Some clinics have a lab to perform the test. Others send the sample to an external lab.

Before the Test

There is no special preparation prior to having a blood smear test. You won't need to restrict eating or activity. It's important to bring your insurance card to your appointment. You should also bring copies of any medical records you were asked to collect.

If you can, bring the results of any other blood tests you've had. Comparing your current results to your past results can be helpful.

During the Test

The technician will begin by finding a vein and cleansing the area with antiseptic. The technician will tie a tourniquet or tight band to your arm to make the vein easier to see. Then they'll insert a needle. You may feel a sharp sting and some pressure as the sample is drawn.

You will be asked to press on the site once the needle is removed. That's to limit bleeding. Then a bandage is applied.

After the Test

You will be able to leave the lab when your blood smear test is done. You'll either return to your treatment room or go home. You'll receive a call with the results. Side effects are uncommon but may include bruising at the site of the blood draw or continued bleeding. In rare cases, infection can happen.

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

Interpreting Results

Blood smear tests can reveal important information about many blood-related conditions. Sometimes a diagnosis can be made based on the blood smear alone, such as with hereditary elliptocytosis, which causes blood cells to have an unusual shape. Other times, further testing will be needed.

The blood smear results offer a "second look" at several findings from a CBC. These include:

  • Red blood cell count
  • Anisocytosis, or differences in red blood cell sizes. Small red blood cells are called microcytes and large red blood cells are called macrocytes. Large cells are often seen with vitamin B12 and folate deficiency. Small cells are often seen with iron deficiency anemia and thalassemia, when your body doesn't make enough red blood cells.
  • The degree of color. Dark red cells have too much color and light red cells don't have enough. 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, ball-shaped spherocytes, and more.
  • Anisopoikilocytosis, or differences in both size and shape of the red blood cells.
  • Presence of inclusions, including parasites
  • Abnormal red blood cells

Advantages of the Blood Smear Test

One of the great advantages of the blood smear over automated tests is the ability to detect findings like these:

  • Burr cells (echinocytes), seen with kidney failure
  • Target cells, seen with abnormal hemoglobins
  • Acanthocytes or spur cells (RBCs with thorny spikes), 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 conditions where the body makes too many red blood cells
  • Schistocytes (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 is stacks of RBCs stuck together, a sign of connective tissue diseases, diabetes, cancer, or an allergic reaction to antibiotics. Rouleaux formation happens with diabetic retinopathy, an eye condition.
  • 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, a vitamin B12 and folate deficiency
  • Heinz bodies or bite cells, seen when a type of hemoglobin clumps in RBCs
  • Cabot's rings (parts of the nucleus that are left over), seen in pernicious anemia and lead poisoning
  • Parasites such as malaria parasites or Bartonella parasites

Blood Smear Tests and White Blood Cells

The specific types of white blood cells counted on a blood smear gives important clues about your health. They include:

  • Lymphocytes, seen with viral infections and some leukemias
  • Neutrophils, seen with bacterial infections, trauma, and some leukemias
  • Eosinophils, seen with allergies, asthma, and parasitic infections
  • Basophils, seen with cancer
  • Monocytes, seen with many health conditions (this type of white blood cell is often compared to a garbage can)

The maturity of white blood cells can show how severe an infection is. It can also indicate leukemia.

  • Bands: These young white blood cells often increase with serious infections.
  • Other immature white blood cells raise concerns about leukemia. This includes finding myeloblasts, metamyelocytes, promyelocytes, or myelocytes. With lymphocytes, it may mean finding lymphoblasts or prolymphocytes.

If your blood smear shows that you have a high number of unusual lymphocytes (more than 5%), you may have mononucleosis.

Other findings include:

  • Toxic granulations in neutrophils, seen with severe infections
  • Hypersegmented neutrophils (more than five nuclear lobes), seen with vitamin B12 and folate deficiency as well as disorders where there are too many red blood cells
  • Bright green inclusions, seen in liver failure
  • Bilobed neutrophils, seen in some genetic syndromes

Apart from WBCs, the number of platelets is important to note, especially if low. Checking the size of the platelets can help narrow down a diagnosis. The blood smear test can also show how many granules are in the platelets. Granules are important in wound healing and inflammation.

Follow-up after a blood smear will depend on what the test shows. For example, if the blood smear results show immature white blood cells, a bone marrow study should be done.

A blood smear is an inexpensive test that can provide a lot of information. While automated tests can be rapid and cost-effective, it's the human eye that still detects many subtle changes in blood cells to help healthcare providers arrive at a diagnosis.

Summary

A blood smear is a test that allows a healthcare provider to take a close look at a blood sample under a microscope. Up close, the smear shows how many of each type of blood cell is present. The sizes, shapes, and colors of the cells can be seen, along with any parasites or fragments in the blood.

The process is simple: A technician draws a small sample of your blood using a needle. The sample is spread onto a slide and then analyzed.

A blood smear helps healthcare providers identify illnesses and find out how well the body is functioning. The results are even more useful when compared to the results of other tests such as CBCs.

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13 Sources
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  1. Alcaraz‐Quiles J, Molina Á, Laguna J, et al. Peripheral blood morphology review and diagnostic proficiency evaluation by a new Spanish EQAS during the period 2011‐2019. Int J Lab Hem. 2021;43(1):44-51. doi: 10.1111/ijlh.13319

  2. Ullah I, Ali MU, Ali S, Rafiq A, Sattar Z, Hussain S. Hematological profile of patients having malaria-positive peripheral blood smears: A cross-sectional study at a diagnostic research center in Khyber Pakhtunkhwa, Pakistan. Cureus. 2018;10(9):e3376. doi:10.7759/cureus.3376

  3. National Library of Medicine. MedlinePlus. Blood.

  4. Adewoyin AS, Nwogoh B. Peripheral blood film - a reviewAnn Ib Postgrad Med. 2014;12(2):71–79.

  5. Beckman AK, Ng VL, Jaye DL, et al. Clinician-ordered peripheral blood smears have low reimbursement and variable clinical value: a three-institution study, with suggestions for operational efficiency. Diagnostic Pathology. 2020;15(1):112. doi: 10.1186/s13000-020-01033-8

  6. Gulati G, Song J, Florea AD, Gong J. Purpose and criteria for blood smear scan, blood smear examination, and blood smear reviewAnn Lab Med. 2013;33(1):1–7. doi:10.3343/alm.2013.33.1.1

  7. Nah EH, M.D, Kim S, et al. Complete blood count reference intervals and patterns of changes across pediatric, adult, and geriatric ages in korea. Annals of Laboratory Medicine. 2018;38(6):503-511. doi: 10.3343/alm.2018.38.6.503

  8. Naghipour Hamzekolaei M, Jafarisani M, Farajzadeh A, et al. Changes in mean platelet volume and hematologic indices in patients with panic disorder due to oxidative stress. Brain Behav. 2020;10(4). doi: 10.1002/brb3.1569

  9. National Center for Advancing Translational Sciences Genetic and Rare Diseases Information Center. Hereditary elliptocytosis.

  10. Nagao T, Hirokawa M. Diagnosis and treatment of macrocytic anemias in adults. J Gen Fam Med. 2017;18(5):200-204. doi: 10.1002/jgf2.31

  11. Doig K, Zhang B. A methodical approach to interpreting the red blood cell parameters of the complete blood count. Clin Lab Sci. 2017;30(3):173-185. doi: 10.29074/ascls.30.3.173

  12. Adewoyin AS, Nwogoh B. Peripheral blood film - a reviewAnn Ib Postgrad Med. 2014;12(2):71–79.

  13. Gorup T, Cohen AT, Sybenga AB, Rappaport ES. Significance of green granules in neutrophils and monocytes. Proc (Bayl Univ Med Cent). 2017;31(1):94-96. doi:10.1080/08998280.2017.1391045

Additional Reading
  • McPherson R., Pincus M., eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 23rd ed. St Louis, MO: Elsevier