Purpose of an Eosinophil (EOS) Blood Test

An eosinophil count (EOS blood test) measures the number of eosinophils, a type of white blood cell, in your body. This result can help identify the cause of certain symptoms and aid in the diagnosis of a number of conditions.

In healthy individuals, eosinophils make up less than 5% of white blood cells. An EOS count greater than this indicates an increased immune response and inflammation, which can point to issues ranging from allergies and autoimmune diseases to certain cancers. Low levels can point to an issue with the production of this cell type.

This article explains when an EOS blood test may be ordered, how you can prepare, and how the test is done. It also discusses what the results may mean and what happens after an EOS blood test.

What Are Eosinophils?

Eosinophils are white blood cells that play an important role in your body’s immune response to help fight off infection. Like other white blood cells, eosinophils are produced in the bone marrow and travel to different tissues throughout the body to protect against infection and disease.

What Is an EOS Test?

An EOS test is a blood test that measures the amount of eosinophils in blood. Eosinophil levels can be measured through a routine complete blood count (CBC) with differential.

The blood differential test measures each type of white blood cell, including eosinophils. Eosinophils and the other white blood cells are measured as counts and as percentages.

Why Would I Need an EOS Blood Test?

Eosinophils increase inflammation to help the body fight off perceived threats (like allergens) and real threats (like infection and disease).

Your healthcare provider may order an EOS blood test if you're experiencing symptoms that indicate you may have abnormal eosinophil levels, such as:

• Chest pain
• Dry cough
• Fever
• General ill feeling
• Rapid breathing
• Rash
• Shortness of breath
• Wheezing

Healthcare providers may recommend this test if they suspect you have a certain condition or disease. There are several for which eosinophil levels may be high (e.g., asthma, parasitic infection, certain cancers) or low (e.g., alcohol use disorder, Cushing's syndrome).

A routine complete blood count (CBC) test is also often part of an annual physical. This blood test will measure your eosinophil level as well as other components in your blood.

While tests that measure eosinophil levels alone cannot be used to identify the existence of a health condition, it can provide information that helps your provider reach a diagnosis.

How an Eosinophil Test Is Done

An eosinophil count is obtained with a straightforward lab test of a blood sample.

1. A healthcare provider finds a vein to draw blood from, most typically on the inside of your elbow.
2. An antiseptic is then applied to the area to kill harmful bacteria.
3. An elastic band is wrapped around your arm to help push blood into the vein, making collecting the sample faster and easier.
4. Once the antiseptic has dried, the healthcare provider or phlebotomist will insert a needle into your vein to collect the blood into a small glass tube (vial) attached to the needle.
5. After the blood has been collected, the elastic band and needle will be removed from your arm.
6. A cotton ball and bandage will be placed on the area to stop bleeding.

From there, your blood will be taken to a lab where it is placed on a microscope slide. A stain is added to the blood to ensure eosinophils are easily detected; they will appear as orange-red granules.

A lab technician will count how many eosinophils are present per 100 cells and share the results with your healthcare provider.

How to Prepare for an EOS Blood Test

There are no special steps required to prepare for the EOS blood test. This includes fasting. However, you may need to fast if you are getting a blood sugar (glucose) test, cholesterol level test (lipid panel), or basic metabolic panel done at the same time.

As with any blood test, a draw may be easier if you are well-hydrated, so drink plenty of water ahead of time.

Tell your healthcare provider about any medications or supplements you are taking before your test. Certain ones can increase eosinophil counts, including:

• Amphetamines, stimulant drugs such as Adderall (dextroamphetamine-amphetamine)
• Antibiotics, used to treat bacterial infections
• Interferons, a type of drug that mimics the signaling proteins your body naturally makes to fight viruses (used for conditions such as melanoma skin cancer, hepatitis, and multiple sclerosis)
• Laxatives containing psyllium, a form of soluble fiber found in products like Metamucil, Fiberall, and Hydrocil, which are sometimes used to ease constipation
• Tranquilizers, drugs used to reduce anxiety and tension, which include Valium (diazepam), Librium (chlordiazepoxide), and Xanax (alprazolam)

What Your EOS Results Mean

The laboratory technician will send the results of your EOS test to your healthcare provider within a week. Your healthcare provider will review the results and share them with you.

Your eosinophil blood counts can vary at different times of day and on different days, but the variability in results is generally not a cause for concern, as this is normal.

Normal Eosinophil Range

A normal absolute eosinophil count ranges from 0 to 500 cells per microliter (<0.5 x 10⁹/L). This typically amounts to less than 5% of all white blood cells.

Different laboratories may have different normal reference ranges. Your healthcare provider can explain your results and provide clarity if you have any questions. 

High Eosinophil Levels

An above-normal eosinophil count is known as eosinophilia. High eosinophil counts can range from mild to severe:

• Mild: 500 to 1,500 eosinophil cells per microliter of blood
• Moderate: 1,500 to 5,000 eosinophil cells per microliter of blood
• Severe: 5,000 or more eosinophil cells per microliter of blood

A high eosinophil count may be due to:

• Allergies (e.g., food, environmental)
• Eczema
• Asthma
• Drug sensitivities
• Bacterial, viral, and parasitic infections
• Autoimmune diseases
• Vasculitis
  
• Adrenal gland deficiency
• Hypereosinophilic syndromes, a group of disorders characterized by high eosinophil counts and organ damage from eosinophilia

As for autoimmune diseases, these can cause eosinophilia:

• Bullous pemphigoid
• Inflammatory bowel diseases (IBDs; Crohn's disease and ulcerative colitis)
• Eosinophilic granulomatosis with polyangiitis (EPGA)
• Eosinophilic myocarditis
• Neuromyelitis optica
• Primary biliary cirrhosis
• Eosinophilic cellulitis (rare cause)
• Eosinophilic fasciitis (rare cause)
• IgG4-related disease (rare cause)

In terms of cancer specifically, the following types can cause high eosinophils:

• Leukemia (adult T-cell leukemia/lymphoma, chronic myeloid leukemia, eosinophilic leukemia)
• Hodgkin's and non-Hodgkin's lymphoma
• Colon cancer
• Ovarian cancer
• Lung cancer

The exact EOS level that may be present with cancer depends on the type of cancer involved and the person it affects. Results can range from slightly high to severely high eosinophil counts.

Low Eosinophil Levels

Eosinophils are normally low in healthy adults. However, certain health conditions and medications may suppress eosinophil levels.

• Corticosteroids and other immune system suppressant medications can reduce the production and function of eosinophils.
• Cushing's syndrome occurs when the body has too much cortisol (stress hormone) over a long period. High levels of cortisol suppress the immune system and may reduce eosinophil counts.
• Sepsis, which is the body's extreme response to an acute bacterial or viral infection, can also cause lower eosinophil levels. This life-threatening condition interferes with the cytokines that regulate eosinophil production.
• Heavy alcohol use lowers white blood cells in general, so it's likely it lowers the amount of eosinophils, too.

What Happens After an EOS Test?

After you get your EOS count, the results will help your healthcare provider in making a diagnosis and determining treatments. Using your medical history, current symptoms, and results as a guide, your healthcare provider may order further tests.

Additional diagnostic tests may involve the cooperation of different specialists. Depending on your situation, these can include hematologists, dermatologists, pulmonologists, gastroenterologists, and neurologists. Once the diagnosis is made, treatment can be tailored accordingly.

Summary

Eosinophils are specialized white blood cells that help the body fight off infection. Your healthcare provider may order an EOS blood test to check the number of eosinophils in the bloodstream.

This may be to investigate the cause of certain symptoms, such as diarrhea, fever, or rash. Or it may be to help diagnose a suspected condition, like asthma, an infection, or certain cancers.

Treatment for the underlying cause of eosinophilia typically restores eosinophil levels within a normal range.