An absolute eosinophil count is a blood test that measures the number of eosinophils, a type of white blood cell, in the body. Eosinophils become active when the body has certain allergic diseases, infections, and other medical conditions. The test is typically ordered as part of a white blood cell count (WBC) with differential, which shows how many of each type of blood cell are circulating in the bloodstream. During the test, a healthcare worker will put a needle into one of the veins and take out some blood. In a lab, a technician will add a special stain to the blood sample, which makes the eosinophils visible, and count how many eosinophils are present in the sample out of 100 cells. That percentage is multiplied by the white blood cell count to give an absolute eosinophil count.

A high eosinophil count can help diagnose a few conditions, such as acute hypereosinophilic syndrome, an allergic disorder like asthma or hay fever, autoimmune conditions, an infection caused by a parasite or fungus, or a reaction to certain medications. A low eosinophil count may be due to alcohol intoxication or overproduction of certain steroids in the body, such as cortisol.

There are no special preparations necessary for this test, but a doctor should be informed if the patient is taking blood-thinning drugs such as warfarin (Coumadin) . Medications that may cause an increased eosinophil count can include diet pills, interferon, some antibiotics, and laxatives that contain psyllium. Risks from having blood drawn are slight, but may include excessive bleeding, fainting or feeling lightheaded, hematoma (blood accumulating under the skin), or infection (a slight risk any time the skin is broken) .