The blood differential test measures the percentage of each type of white blood cell (WBC) that you have in your blood. It also reveals if there are any abnormal or immature cells.

See also:

• Complete blood count (CBC)
• Eosinophil count - absolute

The health care provider will take blood from your vein. The blood collects into an airtight container.

In infants or a young child, blood will be taken from a heel stick or finger stick. The blood is collected in a small glass tube or onto a slide or test strip.

Cotton or a bandage may be applied to stop any bleeding.

A laboratory specialist takes a drop of blood from your sample and smears it onto a glass slide. The smear is stained with a special dye, which helps tell the difference between various types of white blood cells.

Five types of white blood cells, also called leukocytes, normally appear in the blood:

• Neutrophils
• Lymphocytes (B cells and T cells)
• Monocytes
• Eosinophils
• Basophils

A computer or the health care provider counts the number of each type of cell. The test shows if the number of cells are in proper proportion with one another, and if there is more or less of one cell type.

No special preparation is necessary.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

This test is done to diagnose an infection, anemia, and leukemia. It may also be used to see if treatment for any of these conditions is working.

• Neutrophils: 40% to 60%
• Lymphocytes: 20% to 40%
• Monocytes: 2% to 8%
• Eosinophils: 1% to 4%
• Basophils: 0.5% to 1%
• Band (young neutrophil): 0% to 3%

Any infection or acute stress increases your number of white blood cells. High white blood cell counts may be due to inflammation, an immune response, or blood diseases such as leukemia.

It is important to realize that an abnormal increase in one type of white blood cell can cause a decrease in the percentage of other types of white blood cells.

An increased percentage of neutrophils may be due to:

• Acute infection
• Eclampsia
• Gout
• Myelocytic leukemia
• Rheumatoid arthritis
• Rheumatic fever
• Acute stress
• Thyroiditis
• Trauma

• Aplastic anemia
• Chemotherapy
• Influenza
• Widespread bacterial infection
• Radiation therapy or exposure

An increased percentage of lymphocytes may be due to:

• Chronic bacterial infection
• Infectious hepatitis
• Infectious mononucleosis
• Lymphocytic leukemia
• Multiple myeloma
• Viral infection (such as infectious mononucleosis, mumps, measles)
• Recovery from a bacterial infection

A decreased percentage of lymphocytes may be due to:

• Chemotherapy
• HIV infection
• Leukemia
• Radiation therapy or exposure
• Sepsis

An increased percentage of monocytes may be due to:

• Chronic inflammatory disease
• Parasitic infection
• Tuberculosis
• Viral infection (for example, infectious mononucleosis, mumps, measles)

An increased percentage of eosinophils may be due to:

• Allergic reaction
• Cancer
• Parasitic infection
• Hodgkin's disease

A decreased percentage of basophils may be due to:

• Acute allergic reaction

There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

Bagby GC. Leukopenia and leukocytosis. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 173.