SAN DIEGOA 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
Schiffs 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 Schiffs 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.