An absolute eosinophil count is a blood test that measures the number of white blood cells called eosinophils. Eosinophils become active when you have certain allergic diseases, infections, and other medical conditions.

See also: Blood differential

Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore blood flow. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

In the laboratory, the blood is placed on a microscope slide. A stain added to the sample causes eosinophils to show as orange-red granules. The technician then counts how many eosinophils are present per 100 cells. The percentage of eosinophils is multiplied by the white blood cell count to give the absolute eosinophil count.

No special preparation is necessary for adults.

Certain medicines may cause you to have an increase in eosinophils. Such medicines include:

• Amphetamines (appetite suppressants)
• Certain laxatives containing psyllium
• Certain antibiotics
• Interferon
• Tranquilizers

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

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Your doctor may order an eosinophil count if another blood test, called a blood differential is abnormal. It may also be done if the doctor thinks you may have a specific disease.

This test may help diagnose:

• Acute hypereosinophilic syndrome (a rare but sometimes fatal leukemia-like condition)
• An allergic reaction (can also reveal how severe the reaction is)
• Early stages of Cushing's disease
• Parasitic infection

The test may also be done to determine how certain drugs work in your body (such as blood thinners) or to monitor Hodgkin's disease.

Less than 350 cells/mcL (cells per microliter)

High numbers of eosinophils (eosinophilia) are usually associated with allergic diseases and infections from parasites such as worms. A high eosinophil count may be due to:

• Asthma
• Autoimmune diseases
• Eczema
• Hay fever
• Leukemia

A lower-than-normal eosinophil count may be due to:

• Alcohol intoxication
• Over production of certain steroids in the body (such as cortisol)

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

The eosinophil count usually helps confirm, rather than make, a diagnosis. It can not tell if an increase is caused by allergy or parasite infection.

Hoffman R, Benz Jr. EJ, Shattil SJ, et al., eds. Hematology: Basic Principles and Practice. 4th ed. Philadelphia, Pa: Churchill Livingston; 2005:768.

McPherson RA and Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. Philadelphia, Pa: WB Saunders; 2007:474.