Effect of diet on fecal occult blood testing in patients with colorectal polyps.

To determine the effect of a red-meat-free, high-fiber diet, patients performed fecal blood testing before colonoscopy for suspected polyps. One hundred twenty-nine patients were allowed an unrestricted diet, and 159 consumed the special diet. Without dietary restrictions, 31 of 77 patients with adenomas, five of eight with cancer, one of seven with nonneoplastic polyps, and one of 37 with normal colonoscopy had positive tests. With the special diet, 30 of 97 patients with adenomas, eight of ten with cancer, one of six with nonneoplastic polyps, and one of 46 with normal colonoscopy had positive tests. There are no statistical differences between the groups. Comparing all patients with cancer to those with normal colonoscopy, the sensitivity of fecal blood testing is 72%, the specificity 98%, and the positive and negative predictive value 87% and 94%, respectively. The sensitivity for fecal blood testing in patients with adenomas (compared to those with normal endoscopy) is 35%, the specificity is 98%, and the positive and negative predictive values are 97% and 42%, respectively. Combining all patients with adenomas, those with multiple polyps, larger polyps, or polyps located distal to the splenic flexure had positive fecal blood tests more frequently. Fecal blood testing does not detect a large number of colorectal adenomas. This study does not indicate any benefit on the sensitivity or specificity of fecal blood tests from the red-meat-free, high-fiber diet currently recommended.