This adenocarcinoma is arising in a villous adenoma. The surface of the neoplasm is polypoid and reddish pink. Hemorrhage from the surface of the tumor provides for detection with a stool test for occult blood. This neoplasm was located in the sigmoid colon, just out of reach of digital examination, but easily visualized with sigmoidoscopy.


Screening for colon cancer may include a test to detect blood in the stool. The precursor to the fecal occult blood test used today was the "guiacum test" from the 19th century, based upon a color change in chemicals found in the occult blood resin extracted from the tropical Guajacum tree. The heme in hemoglobin provides catalase activity for a color change in the presence of hydrogen peroxide. Many plants have peroxidases.

Currently used guaiac-based fecal occult blood test (FOBT) reagents involve addition of hydrogen peroxide to react with heme, detected by a color change to blue. However, colonic bacteria can enzymatically degrade heme and reduce peroxidase activity. Dietary heme from meat as well as plant peroxidases can interfere with FOBT specificity and sensitivity.

The fecal immunochemical test (FIT) is based upon antibodies specific to the globin in hemoglobin, so a variety of immunoassay methods can detect antibody-globin complexes, even with low concentrations of hemoglobin, and without interference from diet, drugs, or microbial flora. The sensitivity of a single FIT may be as high as 79% for colorectal cancer and 40% for advanced polyps.

Young GP, Symonds EL, Allison JE, et al. Advances in Fecal Occult Blood Tests: the FIT revolution. Dig Dis Sci. 2015;60(3):609-622. doi:10.1007/s10620-014-3445-3

Forbes N, Hilsden RJ, Heitman SJ. The appropriate use of fecal immunochemical testing. CMAJ. 2020;192(3):E68. doi:10.1503/cmaj.190901