At a Glance
Why Get Tested?
To determine if red blood cells (RBCs), white blood cells (WBCs), and platelets are normal in appearance and number, to distinguish between the different types of white blood cells and determine their relative percentages in the blood; a blood smear helps diagnose a range of deficiencies, diseases, and disorders involving blood cell production, function, and destruction; it may also be used to monitor cell production and cell maturity in diseases such as leukemia, during chemo/radiation therapy, and in the evaluation of different types of hemoglobins (hemoglobinopathies).
When to Get Tested?
When complete blood count (CBC) and/or automated WBC differential results are abnormal, a blood smear with a manual WBC differential is performed to determine the presence of and identification of abnormal or immature cells; a doctor may order a peripheral smear differential to evaluate many conditions that affect a patient's red and white blood cells.
A blood sample drawn from a vein in your arm or by pricking a finger, ear or, in the case of an infant, a heel
Test Preparation Needed?
The Test Sample
What is being tested?
A blood smear is a drop of blood spread thinly onto a glass slide that is then treated with a special stain and examined under a microscope by a trained laboratorian. Blood consists of different types of cells suspended in the fluid portion (plasma), and a blood smear is a snapshot of the cells that are present in the blood at the time the sample is obtained. It allows for the evaluation of these cells, which include white blood cells (WBCs), red blood cells (RBCs, erythrocytes), and platelets (thrombocytes).These cell populations are produced and mature in the bone marrow and are eventually released into the bloodstream as needed. The white blood cell's main function is to fight infection, while RBCs carry oxygen to the tissues. Platelets are small cell fragments that are vital to proper blood clot formation. 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. Under the microscope, the stained WBCs can be easily seen and the number and type of cells present can be estimated. One can compare their size, shape, and general appearance to the established appearance of "normal" cells. It is also possible to distinguish between the five different types of WBCs and to determine their relative percentages (manual differential). During this examination, one can also evaluate the size, shape, and color (indicators of hemoglobin content) of the RBCs (RBC morphology) and also estimate 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, red blood cells that appear larger and paler than normal may support other results that indicate a type of anemia. Similarly, the presence of white blood cells 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 sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm or by pricking a finger, ear or, in the case of an infant, a heel.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
Sources Used in Current Review
Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2007, Pp 468-478, 505-516, 539-541,549-559.
(August 4, 2005) Bain B. Diagnosis from Blood Smear, Review article. N Engl J Med 2005; 353:498-507. Available online at http://www.nejm.org/doi/pdf/10.1056/NEJMra043442 through http://www.nejm.org. Accessed February 2011.
(September 9, 2009) Harper J. Pediatric Megaloblastic Anemia. eMedicine article. Available online at http://emedicine.medscape.com/article/959918-overview through http://emedicine.medscape.com. Accessed February 2011.
(September 9, 2009) Artz A. Anemia in Elderly Persons. eMedicine article. Available online at http://emedicine.medscape.com/article/1339998-overview throughhttp://emedicine.medscape.com. Accessed February 2011.
(February 29, 2010) Dugdale D. Blood Smear. MedlinePlus Medical Encyclopedia. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003665.htm through http://www.nlm.nih.gov/medlineplus/ency/article/003665.htm. Accessed February 2011.
Riley R, et.al. How to Prepare and Interpret Peripheral Blood Smear. Medical College of Virginia, Virginia Commonwealth University. Available online at http://www.pathology.vcu.edu/education/PathLab/pages/hematopath/pbs.html through http://www.pathology.vcu.edu. Accessed February 2011.
Sources Used in Previous Reviews
Thomas, Clayton L., Editor (1997). Tabers Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].
Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.
Elstrom, R. (2001 October 28, Reviewed). Peripheral smear. University of Pennsylvania Health System, pennhealth.com [On-line information]. Available online at http://www.pennhealth.com/ency/article/003665.htm through http://www.pennhealth.com.
Biology of Blood. The Merck Manual Home Edition [On-line information]. Available online at http://www.merck.com/mrkshared/mmanual_home/sec14/152.jsp through http://www.merck.com.
Peripheral smear. Robert Wood Johnson University Hospital, Medical Encyclopedia [On-line information]. Available online at http://www.rwjuhh.net/Atoz/encyclopedia/article/003665.asp through http://www.rwjuhh.net.
Cutler, C. (2003 September 14). Blood smear. MEDLINEplus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003665.htm
Brose, M, Updated (2003 May 08, Updated). MEDLINEplus Health Information, Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003657.htm
Blackwell, S. and Hendrix, P. (2001). Common Anemias: What Lies Beneath. Clinician Reviews 11(3):53-62 [On-line journal]. Available online at http://www.medscape.com/viewarticle/436692 through http://www.medscape.com.
(2001 October 15 ). Anemia in Children. American Family Physician, 64:1379-86 [On-line journal]. Available online at http://www.aafp.org/afp/20011015/1379.html through http://www.aafp.org.
Pagana, Kathleen D. & Pagana, Timothy J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 174-176.
Levin, M. (2007 March 9). Blood Differential. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003657.htm. Accessed on 4/10/07.
Dowshen, S. (2007 March). Blood. Nemours Foundation, Teens Health [On-line information]. Available online at http://kidshealth.org/teen/your_body/body_basics/blood.html through http://kidshealth.org. Accessed on 4/10/07.
Vajpayee N, Graham SS, Bem S. Basic examination of blood and bone marrow. Henry's Clinical Diagnosis and Management by Laboratory Methods, 21st ed. Richard McPherson and Matthew Pincus, eds. Saunders Elsievier: Philadelphia. Pp 457-483, 2007.
Bell A, Sallah S. The Morphology of Human Blood Cells, 7th ed. 2005. Abbott, Pp 1-27.