Eosinophilia: when to investigate and what to do in Australian general practice

The finding of an elevated eosinophil count on a routine full blood count is a common GP encounter in Australia — and usually straightforward once the clinical context is clear. Eosinophils are a white blood cell class involved in defence against multicellular parasites and in allergic inflammation. An absolute eosinophil count (AEC) above 0.5 ×10⁹/L is the diagnostic threshold for eosinophilia; counts above 1.5 ×10⁹/L constitute hypereosinophilia and warrant a more structured workup. In Australian general practice, the overwhelming majority of eosinophilia is mild, stable, and attributable to atopic conditions — the same immune pathways that drive asthma, allergic rhinitis, and eczema predictably elevate eosinophil counts. The clinical task is to identify the minority that represent something more: a drug reaction, a parasitic infection, an autoimmune vasculitis, or — rarely — a primary haematological disorder requiring specialist management.

ASCIA and eTG: Antibiotic both provide structured Australian frameworks for this workup, and the BSH 2017 eosinophilia algorithm underpins the international approach that Australian haematology practice follows.

A. Core clinical — the AU general-practice framework

Defining severity

Three severity tiers guide management intensity:

• Mild eosinophilia: AEC 0.5–1.5 ×10⁹/L — the most common presentation in general practice; usually atopic.
• Moderate eosinophilia: AEC 1.5–5.0 ×10⁹/L — requires structured workup; may reflect parasitic infection, drug reaction, connective-tissue disease, or primary haematological process.
• Severe eosinophilia: AEC above 5.0 ×10⁹/L — same-week haematology referral if unexplained; raises concern for hypereosinophilic syndrome (HES) or myeloproliferative neoplasm.

Hypereosinophilic syndrome is defined as persistent AEC ≥1.5 with evidence of end-organ damage — cardiac, pulmonary, neurological, or gastrointestinal — attributable to eosinophil-mediated tissue infiltration.

History that changes management

Eosinophilia workup is guided almost entirely by history. The critical questions are:

Atopic history: asthma, allergic rhinitis, eczema, food allergy, urticaria, anaphylaxis. Stable atopy with mild eosinophilia below 1.5 needs no further investigation in most cases.

Drug history: all current and recently started medications, with particular attention to anticonvulsants (phenytoin, carbamazepine, lamotrigine, valproate), allopurinol, sulfonamides, vancomycin, dapsone, minocycline, and abacavir. DRESS typically appears 2–8 weeks after drug initiation — a timeline that must be checked for every new moderate eosinophilia with rash or liver enzyme elevation.

Travel and residence history: countries visited, freshwater contact (schistosomiasis risk), soil and bare-foot exposure, animal contact, refugee or asylum seeker background, time in endemic regions. Strongyloides may have been acquired decades earlier and persists asymptomatically through auto-infection cycles until an immunosuppressive trigger.

Organ-system symptoms: wheeze or chronic cough (eosinophilic asthma or EGPA), cardiac symptoms including dyspnoea on exertion (eosinophilic myocarditis), peripheral neuropathy or mononeuritis multiplex (EGPA vasculitis), dysphagia (eosinophilic oesophagitis), diarrhoea and abdominal pain (parasitic or eosinophilic colitis), fever with rash (DRESS or vasculitis).

First-line investigations

eTG: Antibiotic and ASCIA align on the initial panel for persistent or moderate eosinophilia:

• Repeat FBC with differential blood film — manual differential distinguishes reactive eosinophilia from dysplastic cells or blasts; confirms persistence across ≥2 measurements at least two weeks apart.
• U&E and LFT — identify hepatorenal involvement in DRESS or HES.
• CRP and ESR — markers of systemic inflammation.
• Total IgE — elevated in atopy, parasitic infection, ABPA, and EGPA; very high levels (>2000 kU/L) suggest parasitic infection or allergic bronchopulmonary aspergillosis.
• Specific IgE or skin-prick testing for suspected allergens, via clinical immunology if complex.
• HIV serology (MBS item 69384) — always in moderate or unexplained eosinophilia.
• TSH — hyperthyroidism can contribute to eosinophilia.
• ANA and ANCA — screen for EGPA (pANCA / MPO-ANCA positive in ~40–60%) and connective-tissue disease.
• Stool ova, cysts, and parasites × 3 over three days — sensitivity improved by multiple specimens; identifies most soil-transmitted helminths.
• Strongyloides IgG serology — essential for any patient with a refugee background, travel to endemic regions, immunosuppression pending, or occupational soil or animal contact. Sensitivity approximately 95% for active infection.

Management algorithm

Treat the identified cause:

• Atopic eosinophilia: optimise the underlying atopic condition. Inhaled corticosteroids for asthma, intranasal corticosteroids for allergic rhinitis, topical therapy for eczema. Annual FBC surveillance is sufficient.
• Parasitic infection: specific antihelminthic per organism from AMH and eTG: Antibiotic. Empirical albendazole plus ivermectin for newly-arrived refugees from endemic regions per RACGP guidelines. Post-treatment FBC at 6–12 weeks to confirm eosinophilia resolution.
• DRESS: stop the offending drug immediately, admit to hospital, commence prednisolone 0.5–1 mg/kg, and monitor LFT, renal function, and eosinophil count daily until trending down.
• Atopy-unrelated moderate eosinophilia: expand workup (imaging, bone marrow if severe) and refer to haematology or immunology depending on phenotype.

B. Evidence — the BSH algorithm in Australian context

The BSH 2017 eosinophilia algorithm is the most widely cited structured approach and is consistent with ASCIA and eTG guidance for the Australian general practice context. Its fundamental distinction — secondary (reactive) versus primary (clonal) eosinophilia — determines the trajectory from GP workup to specialist referral.

Secondary eosinophilia is the overwhelming majority in general practice. Identifying the cause follows from the history: atopy, drug, parasite, autoimmune. Most secondary eosinophilia resolves when the cause is addressed.

Primary clonal eosinophilia — myeloproliferative neoplasms, chronic eosinophilic leukaemia, FIP1L1-PDGFRA-positive HES — is rare but clinically important. FIP1L1-PDGFRA fusion is present in a subset of HES patients and confers dramatic sensitivity to imatinib (complete haematological response in most patients at 100 mg daily). This diagnosis requires bone marrow biopsy with FISH or RT-PCR, available only via haematology referral.

Strongyloides pre-screening before immunosuppression is a non-negotiable safety practice supported by eTG: Antibiotic and RACGP Refugee Health guidelines. Strongyloides hyperinfection syndrome — where corticosteroids or chemotherapy enable massive larval dissemination — carries mortality approaching 80% in published case series when not recognised. Empirical ivermectin before any planned immunosuppression in patients from endemic regions prevents this complication at negligible cost and risk.

C. Causes — the Australian eosinophilia differential

Atopy and allergic disease accounts for most mild eosinophilia seen in Australian clinics. Eosinophil counts in stable atopy are rarely above 1.5 ×10⁹/L. No further investigation is required if the atopic condition is known, managed, and the count is mild and stable.

Parasitic infection is the second most clinically important category, particularly in refugees, recent travellers, and rural Aboriginal and Torres Strait Islander communities. Strongyloides (Strongyloides stercoralis) is the key pathogen because it persists through auto-infection cycles for decades and can cause fatal hyperinfection under immunosuppression. Toxocara (from dog or cat faeces), hookworm, schistosomiasis, and ascaris complete the main Australian-relevant parasites. Threadworm (Enterobius vermicularis) in children does not typically elevate eosinophil counts.

Drug reactions range from simple drug allergy with modest eosinophilia to DRESS — a severe multi-organ syndrome with documented 10–20% mortality. eTG: Antibiotic lists the most common culprit drugs: anticonvulsants, allopurinol, dapsone, sulfonamides, minocycline, vancomycin, and abacavir. Immediate drug cessation is the critical step.

Eosinophilic granulomatosis with polyangiitis (EGPA, formerly Churg-Strauss syndrome) presents in adults with asthma, sinusitis, eosinophilia above 1.5 ×10⁹/L, and new peripheral neuropathy, cardiac involvement, or palpable purpura. ANCA (MPO-ANCA) is positive in approximately 40–60%. EGPA requires urgent rheumatology review and treatment with high-dose corticosteroids and cyclophosphamide or rituximab.

Primary haematological disease — chronic eosinophilic leukaemia, FIP1L1-PDGFRA-positive HES, myeloproliferative neoplasm — is rare but important. Features suggesting primary disease include AEC above 5.0, B symptoms (fever, night sweats, weight loss), splenomegaly, thrombocytopenia or anaemia on FBC, or dysplastic cells on the blood film.

Adrenal insufficiency — often overlooked — causes eosinophilia via loss of cortisol’s suppressive effect on eosinophil release. Unexplained eosinophilia with fatigue, hyponatraemia, and weight loss should prompt a 9 am cortisol and ACTH stimulation test.

D. Australian operations

MBS-rebatable investigations include FBC (MBS item 65070), liver and renal function, CRP, TSH, total IgE, and HIV serology (MBS item 69384). Stool ova, cysts, and parasites are rebatable under the general pathology schedule. Specific IgE testing is rebatable under MBS items 73804–73810 for specified allergens. Bone marrow biopsy (MBS item 73529) and FIP1L1-PDGFRA molecular testing require haematology referral.

PBS antihelminthic access: ivermectin is PBS Authority Required for strongyloidiasis, scabies, and onchocerciasis. Albendazole is PBS-listed for specific helminth indications; private prescription is commonly used for empirical treatment. Praziquantel for schistosomiasis is Authority Required. Imatinib is Authority Required (Written) for FIP1L1-PDGFRA-positive HES. Mepolizumab and benralizumab are Authority Required (Written) under specialist prescription for severe eosinophilic asthma with PBS access through the respiratory pathway.

Refugee health assessment: the RACGP Refugee Health Assessment (MBS items 701 and 705) includes comprehensive parasite screening, empirical antihelminthic treatment, and eosinophilia workup for all newly-arrived refugees and asylum seekers. This is the standard entry point for the majority of parasitic eosinophilia encountered in Australian general practice.

Adverse drug reaction reporting: DRESS should be reported to the TGA Adverse Medicine Events line (1300 134 237). Schistosomiasis is a notifiable condition in most Australian states. Strongyloides hyperinfection deaths are preventable sentinel events and should be reported through local clinical governance processes.

E. Special populations

Refugees and humanitarian entrants: approximately 10–20% of newly-arrived refugees from sub-Saharan Africa, South-East Asia, and the Pacific Islands carry strongyloides by serology. The RACGP Refugee Health guidelines recommend empirical albendazole plus ivermectin at the time of the refugee health assessment, before any planned immunosuppression in the future. This is both a treatment and a safety measure.

Patients about to receive immunosuppression: universal strongyloides serology and empirical treatment before corticosteroids, chemotherapy, biologic therapy, or organ transplant in anyone from an endemic region is a non-negotiable safety step. The hyperinfection mortality justifies this regardless of eosinophil count.

Older adults: unexplained moderate eosinophilia in adults over 50 requires serum protein electrophoresis and serum free light chains to exclude multiple myeloma or lymphoma-related reactive eosinophilia, in addition to the standard panel.

Children: eosinophilia in children is most commonly atopic or post-viral. Paediatric hypereosinophilia with failure to thrive, organomegaly, or recurrent opportunistic infections warrants paediatric haematology or immunology review. Paediatric eosinophilic oesophagitis presents with food refusal, vomiting, and failure to thrive; endoscopic biopsy (≥15 eosinophils per high-power field) is diagnostic.

When to escalate

Refer to haematology urgently (same-week) when:

• AEC above 5.0 ×10⁹/L, especially with cardiac, neurological, or respiratory symptoms suggesting HES.
• Moderate eosinophilia (1.5–5.0 ×10⁹/L) unexplained after initial workup including drug review, parasitology, and serology.
• Blood film shows dysplastic eosinophils, blasts, or features suggesting a myeloproliferative process.
• Rapidly rising AEC on serial counts.

Refer to immunology or rheumatology when EGPA is suspected (asthma + sinusitis + ANCA + neuropathy or cardiac involvement), or when eosinophilic gastrointestinal disease (oesophagitis, gastroenteritis, or colitis) requires endoscopic confirmation and specialist management.

Admit to emergency department immediately when DRESS is identified — stop the drug, start corticosteroids, and admit. Present immediately when strongyloides hyperinfection syndrome is suspected in an immunosuppressed patient with diarrhoea, vomiting, fever, and deteriorating respiratory function.

What this article is and is not

This is general health information drawn from current Australian clinical guidelines — Therapeutic Guidelines (eTG), RACGP Refugee Health, AMH, ASCIA, and the BSH 2017 eosinophilia algorithm. It is not personal medical advice and does not create a doctor–patient relationship. Workup and treatment decisions depend on the full clinical picture and are made collaboratively with your own GP and treating team.

For Australian consumer resources: HealthDirect · Better Health Channel.

For an acute drug reaction emergency, go to the nearest emergency department or call 000. For mental health support: Lifeline 13 11 14.

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Sources cited

1. Therapeutic Guidelines (eTG) — Antibiotic, Tropical Medicine, Respiratory
2. RACGP — Refugee Health guidelines
3. Australian Medicines Handbook
4. ASCIA — eosinophilic conditions
5. PBS — ivermectin, albendazole, imatinib, mepolizumab
6. TGA — Adverse Medicine Events
7. HealthDirect
8. Better Health Channel
9. BSH 2017 — Investigation of eosinophilia in adults
10. Klion — How I treat hypereosinophilic syndrome (Blood 2015)