Eosinophils: what the test measures, normal levels, and what high or low eosinophils mean

Eosinophils are one of the five types of white blood cell counted in a differential. They are made in the bone marrow, circulate briefly, and settle in tissues, mainly the gut lining, the airways and the skin, where they release granules of enzymes and signaling proteins. That arsenal is built for large invaders (parasitic worms) and is also switched on by the allergic pathway, which is why the same cell sits behind hay fever, asthma, eczema, food allergy and drug reactions.

A CBC with differential reports eosinophils two ways. The percentage is their share of all white cells; the absolute eosinophil count is the actual number per microliter of blood, calculated as the total white count multiplied by the percentage. The absolute count is the one that matters, because a percentage moves whenever other white cells move: a person with a low total white count can have a high eosinophil percentage and a perfectly normal absolute number, and the reverse.

Eosinophils also have a strong daily rhythm, running highest late at night and early in the morning and lowest in the afternoon, the mirror image of cortisol, which suppresses them. A dose of prednisone or a surge of stress hormones during an acute illness can drop the count to near zero within hours, so timing and recent illness are part of reading the result.

Most eosinophil counts arrive uninvited, on a complete blood count ordered for another reason, and a mildly high value found that way is the most common reason people look the number up. The count is ordered deliberately when the symptoms fit an eosinophil-driven process: chronic or unexplained itching and rashes, hives, asthma that is hard to control or has a nasal polyp component, chronic sinus disease, food impaction or difficulty swallowing that suggests eosinophilic esophagitis, and persistent diarrhea or abdominal pain, especially after travel.

A new rash, fever or organ inflammation that began within weeks of starting a medication is another trigger for the test, since drug reactions are a leading cause of eosinophilia and the severe form (DRESS syndrome) is defined partly by it. Travel to or residence in regions where intestinal worms are common, exposure to soil or undercooked meat, and adoption or immigration from those regions warrant a count with a low threshold for stool and serologic parasite testing.

People on eosinophil-targeted biologics for severe asthma or eosinophilic conditions (mepolizumab, benralizumab, reslizumab, dupilumab) have counts checked to confirm response, and those with adrenal insufficiency, certain lymphomas or leukemias, or known hypereosinophilic syndrome are followed with it. A normal count in someone who feels well needs no follow-up.

Eosinophils are reported on any complete blood count with differential, which can be ordered by your doctor or online through TestWell for a draw at a Quest or Labcorp patient service center, with a licensed physician reviewing the order and results in your portal within one to three business days. Make sure the order is the version with differential; a plain CBC gives the total white count only.

No fasting is needed. Because eosinophils vary through the day and fall sharply with steroids, note the time of the draw and any corticosteroid use, including inhalers at high doses, nasal sprays, joint injections and steroid creams over large areas, all of which can lower the count enough to hide a mild eosinophilia. A recent infection or an acute stress such as surgery does the same. Tell the ordering physician about new medications in the past two to three months, allergy symptoms, asthma, recent travel and any rash.

If a first result is high, the confirming test is usually drawn at a similar time of day two to four weeks later, off steroids if that is safe, so the two values are comparable and a transient rise from an infection or a medication has had time to settle.

A normal eosinophil percentage is about 1 to 4% of white cells, and a normal absolute count is below 500 cells per microliter (0.5 × 10⁹/L); many healthy people have very few and a count of zero is normal. The absolute number is what is graded. Eosinophilia is a count of 500 or more; it is called mild up to 1,500, moderate from 1,500 to 5,000, and severe above 5,000. The 1,500 line matters most: an absolute count of 1,500 or more on two tests at least a month apart is defined as hypereosinophilia, the threshold at which international consensus and World Health Organization criteria call for a search for organ involvement and an underlying cause.

Mild eosinophilia is very common and, in the United States, is most often allergic. Moderate counts broaden the field to drug reactions, parasites and the less common inflammatory and hematologic causes. Severe counts, or any count with signs of organ involvement (heart, lung, skin, nerve or gut symptoms), are evaluated promptly, because eosinophils themselves can damage tissue when present in large numbers for long enough.

Percentages alone can mislead in both directions. A 10% eosinophil share in a person with a white count of 3,000 is an absolute count of 300, which is normal; a 5% share with a white count of 15,000 is 750, which is eosinophilia. Always convert to the absolute count before reading the result.

Allergic disease leads by a wide margin: allergic rhinitis, asthma, atopic dermatitis, chronic urticaria and food allergy account for most mild eosinophilia in adults and children, and the count often tracks how active the condition is. Medications are next. Almost any drug can do it, but antibiotics (penicillins, cephalosporins, sulfonamides), anti-seizure drugs, allopurinol, NSAIDs and some antiretrovirals are frequent, and a rising count with rash, fever, swollen lymph nodes and abnormal liver tests within two to eight weeks of a new drug is the pattern of DRESS, which needs urgent care.

Parasitic infection is the most common cause worldwide and the first thing to exclude in anyone with travel, immigration or soil exposure: tissue-invasive worms such as Strongyloides, Toxocara, hookworm, schistosomes and filaria raise eosinophils, while protozoa such as Giardia generally do not. Strongyloides deserves special mention because it can persist for decades and become dangerous if corticosteroids are given, so it is tested for before steroids in anyone at risk. Fungal allergic diseases (allergic bronchopulmonary aspergillosis) and skin diseases such as bullous pemphigoid also raise the count.

Less common causes are the ones the 1,500 threshold is designed to catch: eosinophilic esophagitis and gastroenteritis, eosinophilic granulomatosis with polyangiitis (a vasculitis with asthma), adrenal insufficiency (loss of cortisol's suppressive effect), Hodgkin lymphoma and some other lymphomas and leukemias, systemic mastocytosis, and the hypereosinophilic syndromes, including a form driven by a gene fusion (FIP1L1-PDGFRA) that responds to targeted treatment. These are rare, but they are why a persistent count above 1,500 is investigated rather than watched.

There is no meaningful lower limit for eosinophils; a count of zero on a routine differential is a normal finding. Counts fall to near zero under the influence of cortisol, so an acute infection, surgery, trauma, a burst of prednisone or other corticosteroid, or Cushing's syndrome all produce a low count that means only that the stress response or the steroid is present. Alcohol intoxication and some acute bacterial infections lower it too.

A persistently absent eosinophil count is occasionally the first clue to excess cortisol from Cushing's disease when it appears with the right symptoms (weight gain around the trunk, easy bruising, thin skin, high blood pressure, high glucose), and the opposite, an unexpectedly high count, can accompany adrenal insufficiency. Outside those settings, low eosinophils are not a diagnosis and need no follow-up.

Laboratories print both, and the percentage is the one that gets flagged as high most often, which causes needless worry. The percentage is a share of the total white count; it rises whenever eosinophils increase or whenever neutrophils and lymphocytes decrease, and it falls when the total white count is driven up by infection even if eosinophils have not changed. The absolute count is the real number of cells and is what every definition of eosinophilia uses.

To convert: multiply the total white blood cell count (in cells per microliter) by the eosinophil percentage. A white count of 7,000 with 6% eosinophils is 420 per microliter, normal. A white count of 12,000 with 6% is 720, mild eosinophilia. Many reports do the arithmetic for you as "Eos, absolute" or "AEC"; if yours shows only a percentage, do the multiplication before deciding whether anything is abnormal.

A first mildly high count is repeated in two to four weeks, at a similar time of day and, when safe, off corticosteroids, after a review of allergy and asthma control, new medications, recent infections and travel. Most mild elevations either resolve or are explained by an allergic condition at that visit, and the treatment is the treatment of the allergy or asthma itself. A drug that started shortly before the rise is usually stopped or switched, and any rash or fever alongside it is assessed the same day.

A count that stays at 1,500 or above on the repeat triggers a structured evaluation: stool examinations and serology for parasites (Strongyloides and, depending on exposure, Toxocara, schistosomiasis and filaria), a blood smear reviewed by a hematologist, liver and kidney function, troponin and an electrocardiogram to screen the heart, a chest X-ray or CT if there are respiratory symptoms, serum tryptase and vitamin B12, and flow cytometry and molecular testing (including FIP1L1-PDGFRA) when a hematologic cause is possible. The order and depth follow the World Health Organization and international consensus pathways and are directed by a hematologist or allergist.

Same-day care is needed for a high count with chest pain, shortness of breath, palpitations, numbness or weakness, a widespread rash with fever, or swelling of the face and lymph nodes after a new medication. Anyone with eosinophilia and possible Strongyloides exposure should not receive corticosteroids until that infection is excluded or treated.