Usually just a marker of atopy, so common with asthma, hay fever, eczema. Mild generally accepted as 0.5–1.5×10⁹/L, moderate 1.5–5, and severe >5. Persistent would be over 2 weeks.

But beware rare things eg HyperIgE syndrome. Any evidence of organ impairment? Also seen with malignancy, PFAPA, Familial Mediterranean Fever. With the rheumatological conditions, eosinophilia is often severe (>5) but with malignancy, usually mild…

And helminth infection, viz

• Fever and hepatosplenomegaly
• Liver/lung/brain cysts
• Uveitis/macular oedema
• Anaemia with abdominal pain or wheeze

[Turkish study and protocol, not v useful European Journal of Pediatrics (2023); from Florence – https://doi.org/10.1111/apa.17266]