High eosinophils (eosinophilia): causes and next steps

Eosinophils are white blood cells. Patient.info's professional article says their main functions include defence against parasites, allergic responses, tissue inflammation and immunity. They are counted within the white cell differential of a full blood count. A raised count is usually found by chance when a blood test is done for another reason.

The numbers on a report are expressed as cells x 10 to the 9th per litre. NHS Borders considers a count above 0.4 as eosinophilia, and Patient.info subdivides it into mild (0.50 to 1.5), moderate (1.5 to 5.0) and severe (above 5.0). Reference ranges differ a little between laboratories, so compare your result with the range on your own report.

NHS Borders says the most common causes are allergy, eczema or asthma, drug-induced reactions and parasitic infections. Its list of underlying causes to look for also includes hay fever, vasculitis such as eosinophilic granulomatosis with polyangiitis, allergic bronchopulmonary aspergillosis and malignancy, including Hodgkin lymphoma.

Patient.info adds a fuller list: allergic diseases such as asthma, urticaria, eczema and allergic rhinitis; drug hypersensitivity (including the rare DRESS syndrome when a rash and systemic symptoms accompany it); connective tissue diseases such as rheumatoid arthritis; infections, in particular parasitic infections; lymphoma and some leukaemias; adrenal insufficiency; some skin diseases; and states such as after removal of the spleen. It says that many cases have an obvious cause, such as atopic disease, but that the differential includes many serious diseases, including malignancy, which is why a persistent unexplained rise is investigated.

Travel history is important. Patient.info says that a travel history helps assess whether you have been to an area where certain infections, including worm infections, are common. Mention any recent trips, new medicines and any known allergies when you see your GP.

A mildly raised count often causes no symptoms. Patient.info says that when symptoms occur they most commonly involve the skin, lungs and gut, and that low-grade fevers, night sweats, fatigue and weight loss can occur in several of the possible conditions. In other words, the symptoms come from the underlying cause, such as wheeze from asthma or itch from eczema, rather than from the eosinophils themselves.

NHS Borders suggests a full blood count with blood film, CRP, urea and electrolytes, liver function tests, LDH, stool samples for ova, cysts and parasites, and a repeat full blood count at 3 months. Patient.info says that investigation is guided by the history and examination and may include renal and liver function tests, urine tests, and imaging or other tests depending on what the clinician suspects.

If the cause is obvious and the rise mild, such as in a person with known asthma or eczema, further tests may not be needed. If a clear explanation is not found, tests are repeated and, for persistent significant rises, haematology advice is sought. Patient.info notes that a bone marrow biopsy may occasionally be required.

See your GP for a persistently raised result or if you have symptoms such as a rash, wheeze, breathlessness, diarrhoea, unexplained weight loss, fever or night sweats. Sudden severe breathlessness, swelling of the face or throat, chest pain or a spreading rash with fever need emergency help through 999 or NHS 111.

If you have recently started a new medicine and then developed a rash or fever, speak to your GP or pharmacist promptly rather than waiting for a routine review, and do not stop prescribed medicines without advice.

Patient.info says that the outlook varies from mild disease to a fatal outcome depending on the cause, the presence of organ damage, the subtype and the timeliness of treatment, and that tissue damage can occur if eosinophilia is not treated appropriately. For most people with a mild rise due to allergy or a medicine, the count settles once the trigger is treated or removed. The practical message from the guidance is that the cause matters more than the number itself.