Screening for Colon Cancer
Another common early stage screening test is used to detect cancer of the colon. There are actually two tests typically offered for early screening – a fecal occult blood test and a colonoscopy.
Fecal occult blood test (FOBT)
The fecal occult blood test relies on checking a smear of fecal material for the presence of blood. The sensitivity of the test is only 50–60% for one-time use, but may be as high as 90% when it is used every 1–2 years over a long period of time (programmatic sensitivity). Although the sensitivity can be increased using rehydration, this leads to variability in the reaction that invalidates the method as a screening procedure. Low sensitivity leads to a high number of false-negative outcomes, resulting in false confidence.
A colonoscopy involves the insertion of flexible endoscope into the anus in order to check for polyps and growths in the large intestine. Both tests can indicate the presence of small cancers, which can then get treated early, thereby minimizing side effects and increasing the patient’s chances of survival.
While the specificity of the findings with the endoscopic procedure is very high (98–100%), with few false-positives, the sensitivity is low for the entire colon and ranges from 35% to 70%. This is due to the significant number of right-sided adenomas that occur in the absence of distal tumors and are therefore missed on flexible sigmoidoscopy.
Colonoscopy allows for the detection and removal of polyps and biopsy of cancer throughout the colon. Both the specificity and sensitivity of colonoscopy for detecting polyps and cancer are high (at least 95% for large polyps; see below). The miss rate for polyps, on the basis of studies of back-to-back colonoscopies, is 15–25% for adenomas smaller than 5 mm in diameter and 0–6% for adenomas of 10 mm or more.