Colon cancer screening is the most effective weapon in the battle against colon cancer. When the disease is caught at its earliest stages, the cure rate is very high. Screening has led to a decline in the number of deaths from colon cancer over the past 20 years.
Several types of laboratory and clinical tests and examinations are used to diagnose colon and rectal cancer. Most are relatively simple and painless, although some patients do experience minor discomfort requiring mild anesthetics.
The tests include:
- Digital rectal exam
- Fecal occult or hemoccult blood test
- Virtual colonoscopy
- Lower GI series (Barium Enema)
Colonoscopy is the most widely used testing method to study the colon and rectum. It has the highest diagnostic sensitivity and specificity of all available tests, making it the "gold standard" for the diagnosis of colorectal abnormalities. A long, flexible, slender tube, attached to a video camera and monitor, is used to view the inside of the colon. In most studies, 90 to 95 percent of the colon can be examined.
The patient prepares for the procedure by drinking special fluids and/or using laxatives to remove stool from the bowel. Intravenous painkillers and sedation are administered during the examination, which usually takes an hour or less. The test carries a very small risk of complications, including hemorrhage and perforation of the colon wall. If a polyp is found during the procedure, the doctor may remove all or part of it through the scope so that tissues can be examined later for the presence of cancer cells.
In this office exam, the doctor inserts a gloved, lubricated finger into the rectum to feel for large polyps or masses. This procedure is safe and painless (or no more than mildly uncomfortable). But it is limited to the last few inches of the rectum, and it can't detect small polyps. To thoroughly examine the entire colon, other tests are required.
Masses in the colon and rectum are known to bleed early in their development. An inexpensive, easy-to-use screening tool is the guaiac test for occult (hidden) blood in the stool. A small amount of stool is placed on a plastic slide or special paper and examined under a microscope for minute traces of blood.
Patients are asked to eat a lot of fiber and restrict their consumption of red meat, vitamin C, and NSAIDs for several days prior to testing. Since adenomas or tumors bleed intermittently, samples are taken from three successive stool specimens. Positive results indicate the need for complete examination of the colon.
One advantage to this test is that it can be performed at home. However, some polyps and cancers don't bleed, or samples may not be taken when abnormalities are bleeding. So it can give a person with cancer a negative result. Also, the fecal occult blood test detects blood, not cancer. For this reason, among others, some doctors say other tests for colon cancer should be performed in addition to—or instead of—the fecal occult blood test (FOBT).
Another test is stool DNA testing, which finds very early colon cancer by detecting mutated cells that are shed into the stool.
In this test, the doctor inserts a thin, lighted tube called a sigmoidoscope into the rectum to look for polyps, tumors, or other abnormalities. The exam is limited to the rectum and lower colon, but this region of the colon is the site of 60 percent of all tumors. Flexible sigmoidoscopy is often used in conjunction with a fecal occult blood test.
The exam typically takes just a few minutes. Beforehand, you will need to drink plenty of fluids and have an enema to flush stool from the bowel. Sedation is not required for this procedure, which may be performed by general internists and appropriately trained endoscopic technicians. The procedure has a low complication rate with perforation of the colon occurring in only 1 to 2 per 10,000 exams. Since 40 percent of tumors are not located in the rectum or lower colon, the flexible sigmoidoscopy test will not find these tumors.
If a polyp is found during sigmoidoscopy, the doctor may remove all or part of it through the scope so that it can be examined under a microscope for the presence of cancer cells. A polyp is a small abnormal growth in the lining of the colon that often turns into cancer. This procedure is called a biopsy.
This new technology makes use of computed tomography (CT) to get a picture of the colon without actually going inside it. In this procedure, you have a CT scan (a highly sensitive X-ray) of your colon. Sophisticated computer imaging software then manipulates the scan so that your doctor can view all portions of the colon.
Although patients avoid the discomfort and risks of having an instrument inserted into their entire colon, they still must rid their bowels of stool beforehand with enemas, laxatives, and/or special fluids. The physician expands the colon gently by pumping air into it through the rectum. No sedation is required. If a polyp is discovered in a regular colonoscopy, the doctor can immediately remove it. By contrast, if one is discovered with a virtual colonoscopy, the patient must then have a regular colonoscopy.
This is a relatively new test. But research suggests that when experienced doctors use the latest equipment, it is about as accurate as a regular colonoscopy.
A lower GI series (Barium Enema) is a radiological examination of the large intestine, including the rectum. It is sometimes called a barium enema because of the white, chalky substance administered to patients prior to the exam. The barium preparation (a contrast dye) outlines the colon and rectum in the X-rays, which allows the doctor to view tumors or other abnormal tissues. During so-called double-contrast barium enema, the doctor also expands the colon by gently pumping air into it.
Prior to the test, the patient may have to undergo a preparation that includes a liquid diet and enemas or special fluids to clear stool from the colon. The barium preparation is administered through a rectal tube. The test takes about 20 minutes and may involve some discomfort, although no sedation is required.
The entire colon may be examined, though overlapping loops of the bowel may be difficult areas to interpret. The complication rate with the procedure is very low. Because this procedure may miss small tumors, doctors often recommend colonoscopy.
Last reviewed on 7/22/09
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