Association of baseline eosinophil counts with clinical prognosis in cancer patients undergoing immune checkpoint inhibitor therapy

Baseline eosinophil indices can capture host immune tone during immune checkpoint inhibitor (ICI) therapy. We synthesized evidence from 30 studies (4,385 patients) to quantify how baseline absolute (AEC) and relative eosinophil count (REC) relate to outcomes. Compared with lower AEC, high AEC was associated with longer overall survival (OS) and progression-free survival (PFS), higher disease control, and objective response, and increased immune-related adverse events. High REC likewise predicted improved OS and PFS. These results position routine eosinophil measures as low-cost biomarkers for prognostic stratification and for balancing expected benefit with toxicity risk in cancer immunotherapy.