Eosinophil Protein X Test

Fecal Eosinophil Protein X (EPX), also known as eosinophil-derived neurotoxin (EDN), is a protein released by activated eosinophils, immune cells involved in allergic, parasitic, and inflammatory responses. Measuring EPX in stool provides a window into eosinophil activity in the intestinal lining, offering insights into both allergic and inflammatory gut diseases.

About Eosinophils and EPX

Under healthy conditions, eosinophils are present in low numbers in the gastrointestinal (GI) tract, where they help maintain barrier integrity and respond to pathogens. When inflammation occurs, such as in inflammatory bowel disease (IBD), eosinophilic gastroenteritis, or food allergies, eosinophils become activated and release EPX into the intestinal lumen. Because EPX remains stable in feces, its measurement serves as a reliable marker of eosinophilic inflammation within the gut.

Clinical Interpretation

Fecal EPX levels correlate strongly with mucosal eosinophil infiltration and disease activity. In IBD, elevated fecal EPX reflects active inflammation and mucosal injury, with several studies showing it can equal or even surpass fecal calprotectin in detecting active disease. High fecal EPX is also seen in eosinophilic colitis, parasitic infections, and some food allergies. Unlike calprotectin, which reflects neutrophil activity, EPX specifically reflects eosinophil-driven inflammation, making it particularly useful in conditions where eosinophils are the dominant immune cells.

Mildly elevated fecal EPX can occur transiently after viral gastroenteritis, food reactions, or minor mucosal irritation. Persistently high levels are more suggestive of chronic inflammatory or allergic pathology, and repeating the test after symptom improvement or dietary adjustment can help confirm ongoing eosinophilic activity.

Clinical Utility and Monitoring

Because fecal EPX reflects local tissue inflammation rather than systemic eosinophil count, it can detect gut-specific disease activity even when blood eosinophils are normal. This makes it a valuable noninvasive biomarker for monitoring disease flares, assessing treatment response, and distinguishing between eosinophil-predominant and neutrophil-predominant intestinal inflammation.