Blood Smear

Written by Lydia Krause | Published on June 6, 2012
Medically Reviewed by George Krucik, MD

What Is a Blood Smear?

A blood smear is a diagnostic test used to look for abnormalities within the blood. The cell types are examined under a microscope for unusual shapes or sizes. There are three main cells within the blood that the test focuses on:

  • red cells (which carry oxygen throughout the body)
  • white cells (which function as part of the body’s immune system)
  • platelets (which are important for blood clotting)

Abnormalities in the shape, size, and number of the red blood cells can affect how oxygen travels throughout the blood. These abnormalities are often caused by a mineral or vitamin deficiency, but can be caused by abnormal proteins such as in sickle cell anemia.

White blood cells are an integral part of the body’s immune system, a network of tissues and cells that function to keep the body safe from invading microorganisms and eliminate existing infection. Disorders affecting these cells can often result in the body’s inability to eliminate or control infections.

Platelet disorders affect blood clotting and are often the result of the body producing the wrong amount of platelets needed (either too many or too few).

Why Is My Doctor Prescribing This?

This test is prescribed to investigate problems regarding:

  • unexplained jaundice
  • unexplained anemia
  • abnormal bruising
  • persistent flu symptoms
  • sudden weight loss
  • unexpected or severe infection
  • skin rashes or lesions
  • bone pain
  • suspected parasite infection

What Should My Doctor Know Before I Take the Test?

It is essential that you tell your doctor about prescription or non-prescription medications, dietary supplements, and vitamins that you’re currently taking before any medical test is performed.

Anticoagulants such as warfarin, acenocoumarol, and atromentin may affect your test results. Regular blood transfusions, the presence of blood cancer, and disorders like hemophilia will produce abnormalities on the blood smear, so discuss these conditions before the test to avoid a possible diagnostic error.

What Happens During the Test?

The peripheral blood smear is a simple blood test. A phlebotomist, a person trained specifically to draw blood, will tie a tourniquet above the site where the blood will be drawn. He or she will clean and sterilize the injection site with antiseptic and will insert a needle directly into the vein.

Most people feel a sharp pain at the initial poke, which quickly fades as the blood is drawn. Within a few minutes, the needle will be removed and you will be asked to apply pressure to the site with a cotton ball or gauze. A bandage will be placed on the site and you will be free to leave.

A blood test is a low-risk procedure. However, minor risks include:

  • vasovagal syncope (fainting at the sight of blood)
  • dizziness or vertigo
  • soreness or redness at needle site
  • bruising (hematoma)
  • pain
  • infection

What Do Abnormal Results Mean?

Abnormal results in a peripheral blood smear vary by blood cell type affected. Red blood cell type disorders include:

  • iron deficiency anemia
  • vitamin B-12 / folate deficiency
  • sickle cell disease
  • hemolytic-uremic syndrome (an infection in the digestive system produces toxins that destroy red blood cells)
  • polycythemia (increased red blood cells, which can indicate a variety of disorders including polycythemia vera, or may be a factor in diagnosing emphysema, chronic obstructive pulmonary disease (COPD), and other disorders)

White blood cell disorders include:

  • acute leukemia
  • lymphoma
  • human immunodeficiency virus or HIV, in which the virus infects the white blood cells
  • hepatitis C infection
  • malaria
  • parasitic infection (e.g. pinworm)
  • fungal infection (e.g. candidiasis)
  • lymphoproliferative disorders (conditions in which lymphocytes are produced in excessive quantities; e.g. follicular lymphoma, multiple myeloma)

Disorders affecting the blood’s platelets include:

  • myeloproliferative disorders, a group of disorders that cause blood cells to grow abnormally in the bone marrow (e.g. polycythemia vera, chronic myelogenous leukemia)
  • thrombocytopenia, a decrease in platelets, which can be caused by a number of diseases and disorders including vitamin B-12 deficiency, leukemia, systemic viral or bacterial infections, and many others

Other possible findings include:

  • liver disease
  • renal (kidney) disease
  • normal/premature newborn; newborns and premature infants typically have low platelet counts, which generally achieve the normal adult range by about 3 months of age
  • hypothyroidism

What Happens Next?

Discuss your results with your doctor. He or she will be able to advise you if more testing is necessary. If necessary, these are some common tests relating to a peripheral blood smear:

  • CBC
  • G6PD assay
  • bone marrow biopsy
  • hemoglobin variant
  • absolute lymphocyte count
  • delayed hypersensitivity skin test
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Show Sources

  • Abbas, A. K., & Lichtman, A. H. (2011). Introduction to the Immune System; Antigen Recognition in the Adaptive Immune System. In Basic Immunology Updated Edition: Functions and Disorders of the Immune System. (3rd ed.). (pp. 1-21, 66-86). Philadelphia, PA USA: Saunders Elsevier.
  • Bain, B. J. (2005). Diagnosis from a Blood Smear. N Eng J Med, 353(5), 498-507.
  • Jaso, J., Nguyen, A., & Nguyen, A. N. (2011). A Synoptic Reporting System for Peripheral Blood Smear Interpretation. Am J Clin Pathol, 135, 358-364.
  • Kucik, C.J. et al. (2004). Common Intestinal Parasites. Am Fam Physician. Mar 1;69(5):1161-1169. Retrieved on June 14, 2012, at
  • Lynch, E. C. (1990). Peripheral Blood Smear. In Clinical Methods: The History, Physical, and Laboratory Examinations. (3rd ed.). (pp. 732-734). Boston, MA USA: Butterworths.
  • Omalu, I.C. et al. (2008). Standard white blood cell count for malaria density estimation: A need for review? Ann Trop Med Public Health, 1(1), 29-30.

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