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

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.

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.