While histologic assessment is the gold standard in diagnosing EoE, in those patients with a high pre-test probability of EoE and eosinophil count < 15 hpf, tissue staining for eosinophil products, such as eosinophil peroxidase (EPX), might be useful, even though EPX staining remains a research tool [110]. This evidence concerns the gene EPX and eosinophilic esophagitis.