For patients with difficult-to-treat asthma and blood eosinophils ≥300/μl, investigate for non-asthma causes including testing for Strongyloides infection before considering biologic therapy. For patients with hypereosinophilia, causes such as EGPA should be considered and anti-IL4R is preferably avoided as such patients were excluded from the Phase III studies. This evidence concerns the gene IL4R and asthma.