Hepatocyte antigen (Hep Par 1) is helpful in distinguishing between inflamed and architecturally altered ileal and colonic mucosa.

In chronic inflammatory states, the ileal mucosa may flatten, whereas the colonic mucosa may develop a villiform surface. Accordingly, pathologic biopsies labeled by the endoscopist as "ileocolic" or "ileocecal" may generate confusion or uncertainty as to their specific origin. To facilitate distinguishing between architecturally altered ileal and colonic mucosae, we assessed the hepatocyte paraffin 1 (Hep Par 1) antibody, reported to react with normal and metaplastic small bowel epithelium but not with normal colonic epithelium, in 25 ileal biopsies (10 normal and 15 pathologic), 25 colonic biopsies (10 normal and 15 pathologic), and 20 samples labeled as "ileocecal" or "ileocolic" in which the organ of origin could not be definitely established because of mucosal inflammation and distortion. The latter group included 8 cases diagnosed as being of "probable ileal origin," 7 cases diagnosed as being of "probable colonic origin," and 5 cases diagnosed as "uncertain." Diffuse granular cytoplasmic Hep Par 1 expression was detected in all normal and pathologic ileal mucosal biopsies, whereas all colonic biopsies were negative or focally reactive. Cases of "probable ileal origin" were diffusely positive (granular cytoplasmic pattern), whereas those of "probable colonic origin" were negative or focally reactive. Two of the "uncertain" cases expressed Hep Par 1, whereas 3 were negative, thus supporting their ileal and colonic derivation, respectively. In conclusion, Hep Par 1 is diffusely expressed by pathologic ileal mucosa, being negative or only focally positive in pathologic colonic mucosa. Accordingly, it represents a valuable tool for recognizing the tissue source in problematic ileocolonoscopic biopsies.