Eosinophilia Causes and Evaluation
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Direct answer
Eosinophilia is an absolute eosinophil count above 500 per microlitre, and it is most often caused by helminth infection, allergy or drugs. Persistent counts above 1500 per microlitre for more than a month, with organ damage and no secondary cause, define hypereosinophilic syndrome. The evaluation starts with exposure, drug and atopy history, then moves to parasite testing, and only later to a search for clonal disease.
What you must remember
- Definition: an absolute eosinophil count above 500 per microlitre, which must be calculated from the total white cell count rather than read from the percentage alone.
- Helminths: Strongyloides, Ascaris and hookworm are common causes, and filarial infection produces tropical pulmonary eosinophilia with very high counts.
- Allergic causes: asthma, atopic dermatitis and allergic bronchopulmonary aspergillosis, which is usually accompanied by raised total IgE.
- Drug causes: NSAIDs, penicillins, sulfonamides, nitrofurantoin and antiepileptics such as carbamazepine and phenytoin.
- Clonal causes: the FIP1L1–PDGFRA fusion responds to imatinib, and T-cell lymphomas and myeloid neoplasms can also raise the count.
- Hypereosinophilic syndrome: counts above 1500 per microlitre for over a month with organ involvement and no identified cause.
- EGPA: eosinophilic granulomatosis with polyangiitis combines asthma, eosinophilia and small-vessel vasculitis, often with ANCA positivity.
A stepwise evaluation of raised eosinophils
A 40-year-old labourer with nocturnal cough and wheeze has an eosinophil count of 2800 per microlitre and a non-specific chest radiograph. The evaluation moves from the commonest causes to the rarest, guided by the history.
- Take an exposure history: ask about soil contact, bare-foot walking, travel, pets, new drugs and previous atopy, because each points to a different category.
- Check total IgE and stool: high total IgE supports allergy or allergic bronchopulmonary aspergillosis, while stool microscopy and Strongyloides serology define helminth infection.
- Assess the lungs: nocturnal wheeze with a high count suggests tropical pulmonary eosinophilia, which is investigated with antifilarial antibodies and a response to treatment.
- Stop suspect drugs: withdraw the most likely drug and repeat the count, since drug-induced eosinophilia usually settles once the causative agent is stopped.
- Look for organ involvement: cardiac, neurological or cutaneous involvement means hypereosinophilic syndrome must be considered, with echocardiography for eosinophilic cardiac damage.
- Search for clonal disease: when no secondary cause is found and organ damage is present, test for the FIP1L1–PDGFRA fusion and BCR-ABL, and examine the bone marrow.
Where the exam tries to catch you
Eosinophilia questions reward a systematic habit: secondary causes first and clonal disease last.
- Trap: labelling every high count as hypereosinophilic syndrome Correct: the syndrome needs counts above 1500 per microlitre persisting over a month, with organ damage and no other cause.
- Trap: starting corticosteroids before excluding helminths in a patient from an endemic area Correct: steroids can cause Strongyloides hyperinfection, so serology and stool tests come first.
- Trap: assuming imatinib works in every hypereosinophilic patient Correct: imatinib is effective when a PDGFRA or PDGFRB rearrangement such as FIP1L1–PDGFRA is present.
- Trap: reading a raised percentage as a raised count Correct: the absolute count is calculated from the total white cell count, so a modest percentage can still be significant.
Frequently asked questions
What absolute eosinophil count defines eosinophilia?
An absolute eosinophil count above 500 per microlitre, or 0.5 × 10^9 per litre, defines eosinophilia. Counts above 1500 per microlitre for more than a month, with organ involvement, define hypereosinophilic syndrome. Percentages alone can mislead, so the absolute count must always be calculated.
Which helminths most often raise the eosinophil count?
Strongyloides stercoralis, Ascaris lumbricoides, hookworm and filarial worms are the helminths most often linked to eosinophilia. Tropical pulmonary eosinophilia, caused by filarial worms, produces nocturnal cough and very high counts. Microfilariae are often absent from the peripheral blood in this condition.
Why must strongyloidiasis be excluded before starting corticosteroids?
Corticosteroids can trigger hyperinfection or dissemination of Strongyloides stercoralis, which carries a high mortality. Serology and stool examination are therefore done before steroids in patients from endemic areas or with a relevant travel history. Empirical ivermectin is often given when the risk is high.
Which drugs most commonly cause drug-induced eosinophilia?
NSAIDs, penicillins, sulfonamides, nitrofurantoin and antiepileptics such as carbamazepine and phenytoin are frequent causes of drug-induced eosinophilia. Drug reaction with eosinophilia and systemic symptoms, known as DRESS, is the severe form, with rash, fever and organ involvement. Stopping the suspect drug is the key step.
What is tropical pulmonary eosinophilia and how is it treated?
Tropical pulmonary eosinophilia is a filarial-induced syndrome with nocturnal cough, wheeze and very high eosinophil counts. Diagnosis rests on antifilarial antibodies and a good response to treatment, because microfilariae are often absent from the blood. Diethylcarbamazine is the standard antifilarial drug used in its treatment.