Background: Invoking of primary site of carcinoma of unknown origin using immunohistochemistry is essential for accurate diagnosis, particularly in the current era of targeted therapies, smaller sample sizes.
Aim of the work: to assess immunohistochemical expression of CK7, CK20, CDX2 in metastatic colorectal, gastric, pancreatic adenocarcinomas, to evaluate their possible diagnostic role when metastatic colorectal carcinoma suspected in carcinoma of unknown primary site.
Materials, Methods: A retrospective study was performed on 80 paraffin blocks including 40 cases of documented colorectal carcinoma, 20 cases of gastric carcinoma and 20 cases of pancreatic carcinoma were stained by immunohistochemical technique using CK7, CK20, CDX2. The resulted data were statistically analyzed.

Results: CK7-ve/CK20+ve immunoprofile had a specificity of 95% in predicting colorectal adenocarcinomas, which was superior to that of CDX2. CDX2 positivity had a higher sensitivity (95%) than the CK phenotype
Conclusions: Both CDX2 expression, and CK7-ve/CK20+ve are the most sensitive, specific markers for colorectal carcinoma. CDX2 is a useful adjunct for diagnosis of intestinal adenocarcinomas, particularly when CK7, CK20 yield equivocal results. CK7-ve/CK20+ve expression is superior in its specificity, positive predictive value.