Eosinophils are a type of white blood cell. An increased number of eosinophils may mean your body is fighting an infection or allergic reaction. Very high numbers may indicate a serious health condition.

An eosinophil count is a blood test typically reported as part of a white blood cell count (WBC) with differential, when a complete blood count (CBC) is ordered to get a good picture of your overall health. This test shows how many of each type of blood cell are circulating in your bloodstream.

Knowing your eosinophil count can help doctors determine how many eosinophils, a type of white blood cell, are in your bloodstream.

A high number may mean that your immune system is fighting some kind of infection from a virus, bacteria, or fungus or experiencing an allergic reaction. It may also be a sign of another health condition, such as an autoimmune disease.

Eosinophils are a type of white blood cell. Most reside in the gut tissue and survive for up to several weeks. They have a role in defending your body from:

  • fungal infections
  • bacterial infections
  • viral infections
  • parasites, such as worms

Ongoing research continues to uncover an expanding list of roles performed by eosinophils. Nearly every system of the body may rely on eosinophils in some way.

In the immune system, eosinophils destroy invading germs like viruses, bacteria, or parasites, such as hookworms. They also have a role in the inflammatory response, especially if an allergy is involved.

Eosinophils significantly contribute to inflammation related to allergies, eczema, and asthma.

Inflammation helps isolate and control the immune response at an infection site but causes a side effect of tissue damage around it. Allergies are immune responses that often involve chronic inflammation.

White blood cell production

White blood cells are an important part of your immune system. They help protect you from bacteria, viruses, and parasites.

Your bone marrow produces all five kinds of white blood cells. It continually replenishes the white blood cell supply. Each white blood cell lives in the bloodstream for several hours to days.

The number and type of each white blood cell in your body can give doctors a better understanding of your health. Elevated levels often mean your body is sending more white blood cells to fight off infections.

A doctor may discover atypical eosinophil levels through a WBC with differential. A WBC differential test is done as part of a CBC and determines the percentage of each type of white blood cell in your blood. This test will show if you have a high or low count of white blood cells.

White blood cell counts can vary if you have certain diseases or health conditions.

A doctor may also order this test if they suspect specific diseases or conditions, such as:

There are no special preparations necessary for this test. You should inform a doctor if you’re taking blood-thinning drugs such as warfarin (Coumadin). The doctor may advise you to stop taking certain medications before your blood test.

Medications that may cause you to have an increased eosinophil count can include:

Before the test, tell the doctor about any current medications or supplements you’re taking.

A healthcare professional will typically take a sample of blood from your arm by following these steps:

  1. They’ll clean the site with a swab of antiseptic solution.
  2. They’ll insert a needle into your vein and attach a tube to fill it with blood.
  3. After drawing enough blood, they’ll remove the needle and cover the site with a bandage.
  4. They’ll send the blood sample to a laboratory for analysis.

A typical blood sample reading in adults will show fewer than 500 eosinophil cells per microliter (µL) of blood. In children, eosinophil levels vary with age.

If you have over 500 eosinophil cells per mL of blood, a doctor may diagnose a disorder known as eosinophilia.

Healthcare professionals classify eosinophilia into one of three categories. These include:

  • mild (500–1,500 eosinophil cells per cubic millimeter or mm3)
  • moderate (1,500–5,000 eosinophil cells per mm3)
  • severe (greater than 5,000 eosinophil cells per mm3)

Eosinophilia most commonly cause by parasitic infections, but can occur due to any of the following:

Having a very high number of eosinophils can be an emergency. Untreated, it may cause damage to multiple organs.

A low eosinophil count, also known as eosinopenia, can result from intoxication from alcohol or excessive production of cortisol, as in Cushing disease. Cortisol is a hormone naturally produced by the body.

Lower eosinophil counts may also be due to the time of day. Typically, eosinophil counts are lowest in the morning and highest in the evening.

Unless alcohol misuse or Cushing disease is involved, low levels of eosinophils are not usually of concern unless other white cell counts are also abnormally low. If all white cell counts are low, it can signal a problem with bone marrow.

Cushing syndrome is one cause of eosinopenia, involving the overproduction of cortisol by the adrenal glands. If the source of Cushing syndrome is the pituitary gland, this is known as Cushing disease, which can have serious consequences.

A dangerous result of low eosinophil levels may be sepsis, a life-threatening disorder of the immune system.

High levels of eosinophils can occur with many infections, not just parasites, and may lead to certain rare serious conditions, including:

  • eosinophilic asthma, a rare type of asthma where high levels of eosinophils cause inflammation in the respiratory system
  • eosinophilic gastroenteritis, a rare digestive disorder that causes chronic abdominal pain, nausea, diarrhea, and other symptoms
  • eosinophilic esophagitis, an inflammation of the esophagus that causes pain and difficulty swallowing and can be a medical emergency
  • eosinophilic meningitis, a brain and nervous system disorder caused by immune response to parasites
  • eosinophilic leukemia, a type of leukemia characterized by excess eosinophils, often caused by genetic disorders

An eosinophil count uses a standard blood draw.

As with any blood test, there are minimal risks of experiencing minor bruising at the needle site.

In rare cases, the vein may become swollen after blood is drawn. This is called phlebitis. Treat this condition by applying a warm compress several times daily. If this isn’t effective, you should consult a doctor.

Excessive bleeding could be a problem if you have a bleeding disorder or take blood-thinning medication, such as warfarin (Coumadin) or aspirin. This requires immediate medical attention.

If you have an allergy or infection, a doctor may prescribe a short-term treatment to alleviate symptoms and help your white blood cell count reach typical levels.

If you have severe eosinophilia or symptoms that indicate your organs may be affected, a doctor may prescribe high dose corticosteroids, such as methylprednisolone (Depo-Medrol, Solu-Medrol, Medrol).

Doctors may use monoclonal antibodies or immunomodulators if this steroid therapy does not reduce your eosinophil levels.

If your eosinophil count indicates an autoimmune disease, a doctor may want to conduct more tests to determine which type of disease you have. Other conditions can cause high levels of eosinophils, so it’s important to work with a doctor to determine the cause.

Eosinophils are a type of white blood cell. An eosinophil count refers to a blood test ordered as part of a WBC with differential.

A doctor may order this test to get a picture of your overall health. It can help determine if you have an infection or another health condition, such as an autoimmune disease or leukemia.