At a Glance
Why Get Tested?
To evaluate your red blood cells (RBCs), white blood cells (WBCs), and platelets, to distinguish between the different types of WBCs, and to determine their relative percentages in the blood; to help detect, diagnose, and/or monitor a range of deficiencies, diseases, and disorders involving blood cell production, function, and lifespan
When to Get Tested?
When complete blood count (CBC) and/or automated WBC differential results are abnormal or when you have signs and symptoms that a health practitioner suspects are due to a condition affecting your 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. It is a snapshot of the cells that are present in the fluid portion of the blood (plasma) at the time the sample is obtained. The blood smear allows for the evaluation of these cells:
- White blood cells (WBCs, leukocytes) — help fight infections
- Red blood cells (RBCs, erythrocytes) — carry oxygen to tissues
- Platelets (thrombocytes) — small cell fragments that are vital to proper blood clot formation
These cell populations are produced and mainly mature in the bone marrow and are eventually released into the bloodstream as needed. 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. The laboratorian 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, the laboratorian 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, RBCs that appear larger and paler than normal may support other results that indicate a type of anemia. Similarly, the presence of WBCs 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
Gersten, T. (Updated 2012 February 8). Blood smear. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003665.htm through http://www.nlm.nih.gov. Accessed July 2014.
Schick, P. (Updated 2013 February 21). Hemolytic Anemia. Medscape Reference [On-line information]. Available online at http://emedicine.medscape.com/article/201066-overview through http://emedicine.medscape.com. Accessed July 2014.
Curry, C. (Updated 2012 August 1). Differential Blood Count. Medscape Drugs and Diseases [On-line information]. Available online at http://emedicine.medscape.com/article/2085133-overview#showall through http://emedicine.medscape.com. Accessed July 2014.
Lichtin, A. (Updated 2013 September). Evaluation of Anemia. Merck Manual for Healthcare Professionals [On-line information]. Available online through http://www.merckmanuals.com. Accessed July 2014.
Gauer, R. and Braun, M. (2012 May 15). Thrombocytopenia. Am Fam Physician. 2012 Mar 15;85(6):612-622. [On-line information]. Available online at http://www.aafp.org/afp/2012/0315/p612.html through http://www.aafp.org. Accessed July 2014.
(© 1995–2014). Morphology Evaluation (Special Smear). Mayo Clinic Mayo Medical Laboratories [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/print/9184 through http://www.mayomedicallaboratories.com. Accessed July 2014.
(2012 May 18). What is Anemia? National Heart Lung and Blood Institute [On-line information]. Available online at http://www.nhlbi.nih.gov/health/health-topics/topics/anemia/ through http://www.nhlbi.nih.gov. Accessed July 2014.
Lynch, E. (© 1990) Chapter 155 Peripheral Blood Smear. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition. NCBI Bookshelf [On-line information]. Available online at http://www.ncbi.nlm.nih.gov/books/NBK263/ through http://www.ncbi.nlm.nih.gov. Accessed July 2014.
Pagana, K. D. & Pagana, T. J. (© 2011). Mosby's Diagnostic and Laboratory Test Reference 10th Edition: Mosby, Inc., Saint Louis, MO. Pp 168-170.
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
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(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.
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(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.