As promised, the next few blog posts will discuss the risks and benefits of common health screening tests.

Colorectal cancer is the second most common cancer in both men and women in the U.S. Your lifetime risk of dying from colon cancer is 1.94%, which means that 19 people out of 1000 will eventually die from colorectal cancer.

The most commonly recommended screening test for colon cancer is colonoscopy. In this test a long flexible tube with a small camera on the end is passed from the anus all the way to the end of the colon (large intestine). For the test to work well, the colon has to be completely cleaned out of feces, so strong laxatives are given the night before the procedure to clean out the colon. Most people find this to be the most unpleasant part of the test! For the test itself, a sedative is given through a vein because the test would be very uncomfortable without it.

A small instrument can be passed through the tube to remove any polyps that are found. Polyps are small growths on the wall of the colon. Certain kinds of polyps, called adenomatous polyps, have about a 15% chance of eventually turning into cancer but only about fifteen people out of a hundred have this kind of polyp.

So if you have this test, how much does it reduce your chance of dying of colon cancer? Unfortunately, we have no idea! There has never been a good study of colonoscopy in the general population to answer that question. All we know is that, based on one not very good study, people who were at higher than average risk of colon cancer and who had adenomatous polyps had about half as many deaths over 16 years than would have been predicted for the general population.

There is a commonly used rating scale for quality of evidence that goes from A (very good) to D (very poor). This study would get a C at best.

All we can say at present is that for people at average risk for dying from colon cancer, colonoscopy reduces that risk somewhere between 0% (not at all) and probably a lot less than 50%.

What about the risks of getting a colonoscopy?

As we discussed in the previous post, risks for any health screening test include:

1. False positives (the test says you have the disease when you don’t)
2. False negatives (the test says you don’t have the disease when you do)
3. Over diagnosis (you have the disease to such a small degree that it will never harm you)
4. Physical harm from the test itself.

False positives: False positives are not a significant problem with colonoscopy.

False negatives: The false negative rate for colonoscopy is about 10%. This means that one out of every 10 people eventually diagnosed with colon cancer will have had a normal colonoscopy.

Over diagnosis: The smaller the adenomatous polyp is, the less likely that it will eventually turn into cancer. People with very small polyps are at low risk for dying of colon cancer, but are often encouraged to have colonoscopies every three to five years. This exposes them to the risks of colonoscopies that they don’t need. This is over diagnosis.

Physical Harm:

1. Risk of death: a small number of people die from having a colonoscopy. The death rate is about .03%, which means out of every 10,000 people who have a colonoscopy, 3 people will die from it.
2. Risk of perforation (poking a hole in the colon): The risk of perforation ranges from 0.1% to 0.3%, which means that one to three people out 1000 who have a colonoscopy will have a perforation. If this happens it often requires surgery to remove a part of the colon.
3. Hemorrhage (bleeding): the risk of bleeding after a colonoscopy is 0.1% to 0.6%. This means that one to six people out of 1000 who have colonoscopy will have bleeding that requires either repeat colonoscopy, surgery, or some other procedure to stop it.
4. Abdominal pain and bloating: although not life threatening, this can be very uncomfortable. The risk of bloating is 25% and the risk of abdominal pain can be as high as 11%. This means that of 1000 people who have a colonoscopy, 250 will have bloating and 110 will have abdominal pain after the colonoscopy.
5. Gas explosion: This is a very rare, but very dramatic complication. If there is methane gas in the colon from an incomplete cleaning of the colon and oxygen is introduced by putting air in to distend the colon, then use of cautery(an electric current) to remove a polyp can cause an explosion in the colon. There are only a few case reports of this, so we don”t know the exact risk, but it must be very small.

So what is the bottom line?

If you have a higher than average risk of colon cancer because of a family history of colon cancer or some other disease or condition that increases your risk, then it probably makes sense for you to have a colonoscopy. We have one not very good study that supports this.

If you are at average risk, however, your chance of preventing or finding early colon cancer is uncertain, but likely small and the chance of having a complication (and some of them are pretty serious) is probably at least as high or higher than your chance of benefiting.