High Eosinophils on Your Blood Test
From mild allergy to hypereosinophilic syndrome — what your eosinophil count actually means and the investigations that follow.
The Quick Answer
Eosinophils are a type of white blood cell that fights parasites and is heavily involved in allergic reactions. The Australian normal range is roughly 0.04-0.40 x 109/L (1-6% of total white cells). When yours is higher, it's called eosinophilia.
In Australia, the four most common reasons in order are: allergy/asthma, drug reactions, autoimmune disease, and parasitic infections. Mild eosinophilia is rarely an emergency. But counts that stay above 1.5 x 109/L for more than 6 months can silently damage your heart and other organs — so persistent elevation always needs investigation.
What Eosinophils Are and What They Do
Eosinophils are one of the five types of white blood cell, making up just 1-6% of your total white cell count in health. They get their name from the bright pink-orange staining (with the dye eosin) of the granules packed inside them. Those granules contain potent antimicrobial proteins like major basic protein, eosinophil cationic protein, and eosinophil peroxidase.
Their primary jobs are fighting parasitic worm infections (where they're critical) and mediating allergic responses. Eosinophils circulate in the blood for only 8-12 hours before migrating into tissues, where they live for several weeks. The blood count is therefore just a snapshot — tissue infiltration can be much higher than the blood level suggests.
When eosinophils degranulate, they kill not just parasites but also healthy host tissue. This is why sustained eosinophilia damages organs — the same proteins that kill worms also damage heart muscle, lung tissue, nerve sheaths, and skin if released chronically.
Eosinophil Count Categories
The absolute eosinophil count (AEC) is more useful than the percentage. Most labs report both. Treatment urgency depends almost entirely on the AEC and how long it's been elevated.
Normal
Typical causes: No action needed.
Action: Reference range. Eosinophils peak overnight, so morning samples can dip slightly below.
Mild eosinophilia
Typical causes: Allergy (asthma, hay fever, eczema, food allergy), mild parasitic infection, drug reactions, mild atopy.
Action: GP review. Address obvious allergy triggers. Repeat FBC in 6-8 weeks if cause unclear.
Moderate eosinophilia
Typical causes: Drug reactions (DRESS, eosinophilic pneumonitis), parasitic infections, autoimmune disease, eosinophilic gastroenteritis, churg-strauss.
Action: Stop suspected drugs. Stool studies for parasites. Specialist (allergy/immunology, rheumatology) referral. Echocardiogram if persistent.
Severe eosinophilia
Typical causes: Hypereosinophilic syndrome (HES), eosinophilic leukaemia (CEL), lymphoma, helminth infections, severe DRESS.
Action: Urgent haematology referral. Urgent echocardiogram, troponin, brain MRI if symptoms. Bone marrow biopsy often needed.
The Five Categories of Eosinophilia
Doctors think about high eosinophils in five buckets. Working through them in order of likelihood usually identifies the cause in 2-3 GP visits.
Allergic & atopic
Asthma
Often the strongest single cause in Australian adults. Eosinophilic asthma responds particularly well to inhaled steroids and biologics like mepolizumab and benralizumab.
Allergic rhinitis (hay fever)
Especially during pollen seasons. Eosinophil count often correlates with severity of nasal symptoms.
Atopic dermatitis (eczema)
Chronic eczema can produce sustained mild eosinophilia, especially in children.
Food allergy
IgE-mediated food allergies (peanut, egg, shellfish) and eosinophilic oesophagitis can both raise the count.
Drug allergy
Penicillins, sulfas, NSAIDs are common culprits. Mild eosinophilia may be the only sign.
Parasitic infections
Strongyloides stercoralis
Endemic in northern Australia and Indigenous communities. Can persist for decades. Important to exclude before any immunosuppression.
Hookworm (Ancylostoma, Necator)
Common in tropical Australia and after travel. Causes iron deficiency anaemia plus eosinophilia.
Schistosomiasis
After travel to Africa, the Middle East, or parts of South America/Asia. Can cause chronic eosinophilia for years.
Toxocara canis/cati
From dog or cat faeces, especially in children. Causes visceral larva migrans with high eosinophil counts.
Trichinella spiralis
From undercooked pork or wild game. Causes muscle pain plus eosinophilia.
Echinococcus (hydatid)
From sheep-farming areas. Cysts in liver/lung, often asymptomatic until they rupture.
Drug reactions
Penicillins and cephalosporins
The single most common cause of drug-induced eosinophilia. Usually mild and resolves on stopping.
Sulfa drugs (TMP-SMX, sulfasalazine)
Can cause mild eosinophilia or full DRESS syndrome (Drug Reaction with Eosinophilia and Systemic Symptoms).
Allopurinol
Most dangerous in patients of Han Chinese, Thai, or Korean ancestry due to HLA-B*5801 association. Can cause severe DRESS.
Anticonvulsants (phenytoin, carbamazepine, lamotrigine)
Classic DRESS triggers. Onset 2-8 weeks after starting the drug. Always discontinue if eosinophilia plus rash develops.
NSAIDs
Can cause eosinophilic pneumonitis and interstitial nephritis as well as simple drug allergy.
Proton pump inhibitors
Increasingly recognised cause of mild persistent eosinophilia and acute interstitial nephritis.
Autoimmune & inflammatory
Eosinophilic granulomatosis with polyangiitis (EGPA, Churg-Strauss)
Vasculitis affecting people with adult-onset asthma. Eosinophils above 1.5 x 10^9/L plus asthma, neuropathy, sinusitis. Treated with steroids and immunosuppression.
Eosinophilic gastroenteritis
Eosinophil infiltration of the gut wall. Causes abdominal pain, diarrhoea, weight loss. Diagnosed on biopsy.
Eosinophilic oesophagitis
Increasingly common cause of food impaction in young men. Diagnosed on endoscopic biopsy showing more than 15 eosinophils per high-power field.
Inflammatory bowel disease
Crohn disease and ulcerative colitis can produce mild eosinophilia.
Adrenal insufficiency
Cortisol normally suppresses eosinophils; lack of cortisol (Addison disease, hypopituitarism) causes mild eosinophilia.
Haematological & malignant
Hypereosinophilic syndrome (HES)
Persistent eosinophilia above 1.5 x 10^9/L for more than 6 months with end-organ damage (heart, lungs, nervous system, skin). Requires haematology referral.
Chronic eosinophilic leukaemia (CEL)
Rare clonal disorder. Often associated with FIP1L1-PDGFRA fusion gene. Responds dramatically to imatinib if PDGFRA-positive.
Hodgkin and non-Hodgkin lymphoma
Reactive eosinophilia is common in Hodgkin lymphoma. Always examine for lymphadenopathy in unexplained eosinophilia.
Solid organ tumours
Lung, gastric, pancreatic and colorectal cancers can produce paraneoplastic eosinophilia, usually mild.
Mast cell disease (mastocytosis)
Often causes flushing, urticaria, and tryptase elevation alongside eosinophilia.
Symptoms That Can Accompany High Eosinophils
Many people with mildly raised eosinophils have no symptoms at all. When symptoms occur, they often point to the underlying cause. The symptoms below should be specifically asked about by your GP.
Wheeze, breathlessness, cough
Eosinophilic asthma is the most common cause. Often worse at night or after exercise. Inhaled steroids dramatically reduce both symptoms and eosinophil count.
Itchy skin, hives, eczema
Atopic dermatitis and chronic urticaria both elevate eosinophils. The itch is often worse at night.
Nasal congestion, postnasal drip
Allergic rhinitis with or without nasal polyps. Saline rinses and intranasal steroids help.
Diarrhoea, abdominal cramps
Eosinophilic gastroenteritis or parasitic infection. A stool sample is the first test.
Difficulty swallowing or food sticking
Eosinophilic oesophagitis, especially in young men. Gastroscopy with biopsies is needed for diagnosis.
Numbness, tingling, weakness
Eosinophilic neuropathy or vasculitis. Always concerning — consider EGPA (Churg-Strauss).
New rash with fever
DRESS syndrome until proven otherwise. Stop any new drug and seek urgent review.
Chest pain or palpitations
Eosinophilic myocarditis or endomyocardial fibrosis. Get an echocardiogram and troponin.
Red Flags — Don't Wait
Most cases of mildly elevated eosinophils can wait for a routine GP appointment. But these specific findings need urgent investigation:
Sustained eosinophil count above 1.5 x 10^9/L for over 6 months
This is the threshold for hypereosinophilic syndrome. Even without symptoms, eosinophils at this level can silently damage the heart and other organs.
New rash with eosinophilia and fever
DRESS syndrome (Drug Reaction with Eosinophilia and Systemic Symptoms). Discontinue the suspected drug immediately and seek urgent medical review.
Asthma plus high eosinophils plus nerve symptoms
Triad suggesting eosinophilic granulomatosis with polyangiitis (EGPA, Churg-Strauss). Requires urgent rheumatology referral.
Chest pain or breathlessness with high eosinophils
Eosinophilic myocarditis or eosinophilic pneumonia. Both are medical emergencies. Get to ED.
Stroke-like symptoms with high eosinophils
Eosinophilic embolic events to the brain. Rare but serious complication of HES.
High eosinophils plus enlarged lymph nodes or spleen
Suggests lymphoma or chronic eosinophilic leukaemia. Needs haematology referral.
High eosinophils after travel to tropics
Specific parasite serology and stool studies needed. Some parasites need specific drugs and can cause delayed complications.
Eosinophils above 5.0 x 10^9/L on any single test
Always abnormal and requires investigation. Repeat FBC immediately and arrange urgent specialist review.
What Your GP Will Investigate — Step by Step
Confirm and quantify
Repeat the FBC to confirm the elevation is genuine and not a lab artefact. Calculate the absolute eosinophil count (AEC) from the differential — this is more useful than the percentage. Note any trend over previous tests.
Take a focused history
Travel history (where, when, what activities), medication and supplement list (including over-the-counter and herbal), allergy and asthma history, dietary changes, occupation, pet exposure, and any family history of atopy or autoimmune disease.
Allergy and asthma assessment
Spirometry if any respiratory symptoms. Total IgE plus specific IgE testing (RAST or skin prick) for common Australian allergens (dust mite, pollens, animal dander, common foods). Address obvious allergy triggers first.
Parasite screening
Stool microscopy on three separate samples (parasites are shed intermittently). Specific serology for strongyloides, schistosoma, and toxocara if travel or exposure history. PCR-based stool tests are increasingly used in major Australian labs.
Drug review
Stop any non-essential medication started in the last 2 months. Recheck FBC in 4-6 weeks. Drug-induced eosinophilia usually resolves within 2-4 weeks of stopping the offending drug.
Inflammation and autoimmune workup
CRP, ESR, ANCA (for vasculitis), tryptase (for mast cell disease), IgE level, vitamin B12 (raised in myeloproliferative disorders), LDH (raised in lymphoma), and a thoroughly examined blood film.
Imaging if persistent or symptomatic
Chest X-ray for pulmonary infiltrates, ultrasound abdomen for hepatosplenomegaly or lymphadenopathy. CT chest/abdomen/pelvis if lymphoma suspected. Echocardiogram for any persistent eosinophilia above 1.5 x 10^9/L.
Specialist referral
Haematology referral for unexplained moderate or severe eosinophilia. Allergy/immunology referral for allergic causes that need biologics. Rheumatology referral if autoimmune cause suspected. Bone marrow biopsy may be needed for unexplained sustained eosinophilia.
Treatment — What Lowers Eosinophils
Allergen avoidance
Identifying and avoiding the trigger (foods, dust mite, pets, mould, pollen) is the first-line treatment for allergic eosinophilia. Often takes weeks for the count to fall.
Inhaled corticosteroids
For asthma and eosinophilic airway disease. Powerful eosinophil suppressors. Typical drugs: fluticasone, budesonide, beclomethasone.
Oral corticosteroids
Reserved for moderate-to-severe eosinophilic disease. Prednisolone 0.5-1 mg/kg/day usually drops eosinophils within 24-48 hours. Side effects limit long-term use.
Stop the offending drug
For drug-induced eosinophilia, stopping the drug usually normalises eosinophils within 2-4 weeks. DRESS syndrome may need oral steroids in addition.
Antiparasitic treatment
Albendazole, ivermectin, praziquantel depending on the parasite. Strongyloides treatment is critical before any immunosuppression.
Biologics for eosinophilic asthma
Mepolizumab, benralizumab, and reslizumab target IL-5 (the main eosinophil growth factor). Dramatically reduce eosinophils and asthma exacerbations. Available on PBS for severe eosinophilic asthma.
Imatinib for FIP1L1-PDGFRA-positive CEL
Targeted therapy that produces complete remission in this specific haematological subtype. Tested for via FISH or PCR on blood or bone marrow.
Hydroxyurea or interferon-alpha
Used in hypereosinophilic syndrome when steroids fail or are not tolerated. Both reduce eosinophil production by the bone marrow.
Related Reading
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This page provides general educational information about elevated eosinophils. It is not a substitute for professional medical advice, diagnosis, or treatment. Persistent or severe eosinophilia requires evaluation by a GP and often a haematologist or immunologist. SmarterBlood does not provide medical care.