Quality assurance is a particular issue with guaiac fecal occult blood testing (gFOBT): Sensitivity and specificity appear highly dependent on brand, sample collection, preparation, and interpretation. Some older variants (e.g., unrehydrated Hemoccult II) miss most cancers and are not recommended.
Fecal immunochemical testing (FIT) is as sensitive as gFOBT but considerably more specific: False positives related to diet and vitamin C are unlikely. Collection techniques (fewer stool samples, less handling) may make some variants of FIT more acceptable to patients.
When considered several years ago by the organizations that sponsored the Guideline, sDNA testing, like CTC, was “felt to be promising but with insufficient evidence to recommend for screening,” Dr. Brooks notes. Subsequent research and steady improvements in technology “show pretty clearly that both are at least equivalent to existing tests.”
sDNA testing operates on a novel principle: Rather than occult blood, the test detects molecular markers associated with neoplasia. Specificity is high (93%-97%), but since the test can target only a limited panel of abnormalities (21 separate point mutations in the original version), it is bound to miss some lesions. Test sensitivity has actually been comparable to that of gFOBT. The recommended frequency of sDNA testing has not yet been established.
The Guideline emphasizes the importance of proper sample collection and follow-up for stool testing. In a recent national survey, nearly one third of PCPs reported collecting stool samples during digital rectal examination (which reduces gFOBT sensitivity to 9%), and a similar proportion said they followed up positive gFOBT findings by repeating the test or referring for sigmoidoscopy, rather than suggesting colonoscopy, which is recommended.
Colonoscopy “tends to be the standard against which other tests are measured because it is the only one that allows direct visualization of the entire colon wall,” Dr. Brooks says. “But it’s a misconception to view it as the ‘gold standard’; that would imply a level of perfection colonoscopy doesn’t achieve.” It fails to detect 5% of cancers and 6%-12% of large adenomas.
Clinicians performing the procedure vary considerably in training and skill. When referring patients, PCPs should look for consultants who report factors that can influence accuracy, such as thoroughness (mention of the cecum indicates the entire colon was viewed) and time, Dr. Brooks says. One large well-designed study found significantly more missed abnormalities when withdrawal time was under six minutes.
The polyp detection rate can be another quality indicator. In most screening populations, an incidence of 20%-25% is to be expected.
In experienced hands, the sensitivity of CTC (“virtual colonoscopy”) for cancer and large adenomas appears comparable to that of optical colonoscopy, according to the Guideline.
One drawback of CTC is the need for optical colonoscopy to biopsy or remove lesions that are discovered—a second procedure that will again require full bowel preparation. CTC should be repeated every five years, vs. 10 years for colonoscopy, the Guideline says.
As a practical matter, the availability of CTC is still “extraordinarily limited,” Dr. Brooks notes. As it expands beyond research centers and urban settings, “you want to be certain the individuals performing and reading tests are well trained and use up-to-date equipment.”
Sigmoidoscopy and double-contrast barium enema remain recommended options, although usage of these procedures, both of which are less sensitive than colonoscopy, has declined substantially in recent years.