WBC Differential

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Also known as: Leukocyte Differential Count; Peripheral Differential; WBC Count Differential; Diff
Formal name: White Blood Cell Differential

A WBC differential typically includes the following: absolute neutrophil count or % neutrophils, absolute lymphocyte count or % lymphocytes, absolute monocyte count or % monocytes, absolute eosinophil count or % esosinophils, and absolute basophil count or % basophils.

At a Glance

Why Get Tested?

To help determine the cause of abnormal results on a WBC count; to help diagnose and/or monitor an illness affecting your immune system, such as an infection or inflammatory condition, or cancers that affect your white blood cells, such as leukemia

When to Get Tested?

As part of a complete blood count (CBC), when you have a routine health examination; when results of a CBC fall outside the reference range; when you have any number of signs and symptoms that may be related to a condition affecting white blood cells, such as infection, inflammation, or cancer; when you have a condition or are receiving treatment that is known to affect WBCs

Sample Required?

A blood sample drawn from a vein in your arm or by a fingerstick (children and adults) or heelstick (infants)

Test Preparation Needed?

None

The Test Sample

What is being tested?

White blood cells (WBCs), also called leukocytes, are cells that circulate in the blood and the lymphatic system that help protect the body against infections. They are an important part of the body's immune system and also have a role in inflammation, allergic responses, and protection against cancer. There are five types of white blood cells, each with different functions. A WBC differential totals the number of each of the different types of WBCs in a person's sample of blood. The differential reveals if the cells are present in normal proportion to one another, if the number of one cell type is increased or decreased, or if abnormal and/or immature cells are present. This information is helpful in diagnosing specific types of illnesses that affect the immune system and the bone marrow.

A differential may be performed in conjunction with a complete blood count (CBC), a test often used as a general health check, or it may be performed in follow-up to abnormal results on a CBC. Most often, a differential is performed on an automated blood analyzer, but also may be performed manually by a trained laboratorian who examines a blood smear under a microscope. The values are typically reported as absolute numbers of cells but may be expressed as the relative percentages of the total number of WBCs.

White blood cells develop from precursor cells produced in the bone marrow. The five different types of WBCs include:

  • Granulocytes—these white blood cells have granules in their cytoplasm. The granules contain chemicals and other substances that are released as part of an immune response. The three types of granulocytes include:
    • Neutrophils (neu) normally make up the largest number of circulating WBCs. They move into an area of damaged or infected tissue, where they engulf and destroy bacteria or sometimes fungi. Young neutrophils, recently released into circulation, are called bands.
    • Eosinophils (eos) respond to infections caused by parasites and play a role in allergic reactions (hypersensitivities)
    • Basophils (baso) usually make up the fewest number of circulating WBCs and are thought to be involved in allergic reactions
  • Lymphocytes (lymphs) exist in both the blood and the lymphatic system. They are divided into three types, but the differential does not distinguish among them. All lymphocytes differentiate from common lymphoid stem cells in the bone marrow. The differential counts and reports all lymphocytes together. Separate specialized testing must be done to differentiate the three types:
    • B lymphocytes (B cells) are antibody-producing cells that are essential for acquired, antigen-specific immune responses. Fully mature B-cells are called plasma cells that produce antibodies, immune proteins that target and destroy bacteria, viruses and other "non-self" foreign antigens.
    • T lymphocytes (T cells) finish maturing in the thymus and consist of a few different types. Some T cells help the body distinguish between "self" and "non-self" antigens. Others initiate and control the extent of an immune response, boosting it as needed and then slowing it as the condition resolves. Other types of T cells directly attack and neutralize virus-infected or cancerous cells.
    • Natural killer cells (NK cells) directly attack and kill abnormal cells such as cancer cells or those infected with a virus.
  • Monocytes (mono), similar to neutrophils, move to an area of infection and engulf and destroy bacteria. They are associated more often with chronic rather than acute infections. They are also involved in tissue repair and other functions involving the immune system.

Normal blood smearNormal blood smear showing a lymphocyte and a monocyte

When there is an infection or an inflammatory process somewhere in the body, the bone marrow produces more WBCs, releasing them into the blood. Depending on the cause of infection or inflammation, one particular type of WBC may be increased as opposed to other types. As the condition resolves, the production of that type of WBC subsides and the number drops to normal levels again.

In addition to infections and inflammation, there are a variety of conditions that can affect the production of WBCs by the bone marrow or their survival in the blood, resulting in either increased or decreased numbers. The differential, along with the other components of the CBC, alerts the doctor to possible health issues. Results are often interpreted in conjunction with additional tests such as a blood smear review, which can reveal the presence of abnormal and/or immature populations of WBCs.

In a few serious diseases, some immature forms of the cells are released from the bone marrow into the circulation and may be detected by the WBC differential. This may occur with bacterial infection, leukemia, myelodysplastic disorders, or myeloproliferative neoplasms, for example. Some immature cells that may be detected include metamyelocytes, myelocytes, promyelocytes, and/or blasts.

If results indicate a problem, a wide variety of other tests may be performed in order to help determine the cause. A doctor will typically consider an individual's signs and symptoms, medical history, and results of a physical examination to decide what other tests may be necessary. For example, as needed, a bone marrow biopsy will be performed to evaluate the bone marrow status.

How is the sample collected for testing?

A blood sample is drawn from a vein in the arm or from a fingerstick (for children and adults) or heelstick (for infants).

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.

The Test

Common Questions

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Article Sources

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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

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Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2007, Pp 547-559.

Wintrobe's Clinical Hematology. 12th ed. Greer J, Foerster J, Rodgers G, Paraskevas F, Glader B, Arber D, Means R, eds. Philadelphia, PA: Lippincott Williams & Wilkins: 2009, Pp 170-402, 1528-1533.

(Feb 1 2011) National Heart Lung Blood Institute. What is lymphocytopenia? Available online at http://www.nhlbi.nih.gov/health/health-topics/topics/lym/ through http://www.nhlbi.nih.gov. Accessed April 2012.

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(Oct 25 2010) American Cancer Society. Non-Hodgkin Lymphoma. Available online at http://www.cancer.org/Cancer/Non-HodgkinLymphoma/DetailedGuide/non-hodgkin-lymphoma-what-is-non-hodgkin-lymphoma through http://www.cancer.org. Accessed April 2012.

(January 8 2010) Kempert P. White Blood Cell Function, Overview of the Immune System. Medscape Reference article. Available online at http://emedicine.medscape.com/article/960027-overview through http://emedicine.medscape.com. Accessed April 2012.

(January 26, 2010) Naushad H. Leukocyte Count (WBC). Medscape Reference article. Available online at http://emedicine.medscape.com/article/2054452-overview#aw2aab6b2 through http://emedicine.medscape.com. Accessed April 2012.

(Updated December 5, 2011) Nader N. Neutrophilia. Medscape Reference article. Available online at http://emedicine.medscape.com/article/208576-overview through http://emedicine.medscape.com. Accessed April 2012.

(May 24, 2011) Godwin J. Neutropenia. Medscape Reference article. Available online at http://emedicine.medscape.com/article/204821-overview through http://emedicine.medscape.com. Accessed April 2012.

(August 25, 2011) Liss M. Eosinophilia. Medscape Reference article. Available online at http://emedicine.medscape.com/article/199879-overview through http://emedicine.medscape.com. Accessed April 2012.

Sources Used in Previous Reviews

Thomas, Clayton L., Editor (1997). Taber's 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.

Susan J. Leclair, PhD, CLS(NCA). Professor of Medical Laboratory Science, Department of Medical Laboratory Science, University of Massachusetts, Dartmouth, MA.

Pagana, Kathleen D. & Pagana, Timothy J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 1003-1008.