SAN DIEGO—A new type of colorectal cancer screening test appears to be as sensitive as fecal occult blood testing (FOBT) with higher specificity, according to a Canadian study. Norman E. Marcon, MD, of the Centre for Therapeutic Endoscopy and Endoscopic Oncology, Toronto, reported results of a study comparing the new test, ColorectAlert, with FOBT at an American Gastroenterological Association research forum held during the Digestive Disease Week conference.
ColorectAlert determines the presence of GAG, a carbohydrate moiety that has been associated with adenocarcinomas, in rectal mucus samples. “ColorectAlert makes use of a novel color measurement to quantitate the amount of GAG present in a rectal mucous sample,” he said.
The mucus samples were obtained during a digital rectal exam prior to the colonoscopy and smeared onto Colo-rectAlert membranes, which were sent to the lab for testing. The results were determined by treating the mucus samples with galactose oxidase and then staining with Schiff’s reagent. The results were quantitated by determining the color of the developed sample, using a hand-held spectrophotometer.
The samples were then treated with periodate to oxidize all the carbohydrate residues and stained again with Schiff’s reagent before being tested again for color. Test results were considered positive if the measured color was greater than 370 and negative if less than 350. Samples with a value between the two numbers after a periodate treatment of less than 350 were also considered positive.
Researchers also obtained stool samples from three consecutive bowel movements for fecal occult blood testing.
“We compared ColorectAlert results with FOBT in 601 patients scheduled for a colonoscopy and who had provided informed consent,” Dr. Marcon said. “But the testing was done without the knowledge of the colonoscopy results.”
Based on the colonoscopy, 40% of the participants had a normal bowel, 20% had benign bowel disease, 34% had polyps, and 4% had cancer. Both FOBT and ColorectAlert detected 81.3% of the cancers, but the odds ratio associated with a positive ColorectAlert test was 13.1, compared with 5.6 for FOBT.
“FOBT and ColorectAlert were equally sensitive for cancer detection,” Dr. Marcon said, “but ColorectAlert was significantly more specific than FOBT in this particular study population (75% vs 57%). Most lesions we saw were clustered in the rectum and sigmoid.”
The researchers still have some questions that need answering, Dr. Marcon noted. “How does bowel preparation for colonoscopy affect ColorectAlert results?” he asked. “Are the tests consistent over time? Do ColorectAlert values change in response to therapy? And how does the test perform in a low-risk population?” But, he said, “apparently the higher specificity of ColorectAlert may make it more cost-effective than FOBT because fewer colonoscopies would be necessary.”
