As noted in the previous post, 19 people out of 1000 will eventually die from colorectal cancer.  In this post I will discuss another screening test for colorectal cancer, checking stools for tiny amounts of blood.

You do this test at home once a year.  Your doctor will give you a kit to do the test. The kit contains three small disposable wooden sticks and 3 cards connected together.  When you have a bowel movement, you open one of the cards, reach down into the toilet with one of the sticks and get a small amount of stool to smear on the special areas on the card, and then close the flap on the card.  You do the same thing for two more bowel movements on two different days for the other two cards. The cards usually come with a special sealable pre-addressed envelope so that you can mail the cards back to your doctor’s office.  Your doctor’s office staff opens the other side of the card and puts special developing solution on the card.  If it turns blue, then there is some blood in the stool smear. If the test is positive, then a colonoscopy is recommended. Certain foods, such as horseradish and certain raw vegetable can cause a false positive test. Lots of meat or vitamin C can also cause false positives. These foods should be avoided the day before the three days of testing.

Unlike colonoscopy, there has been very extensive testing of the effectiveness of FOBT in reducing the risk of death from colon cancer. If you do this test every year, your risk of death from colorectal cancer is reduced anywhere from 16% to 27% in several large randomized trials. This means that there would be three to five less deaths per 1000 people.  In other words the 19 per thousand death rate would be reduced to 14 to 16 deaths.

Once again, if you are at average risk your chance of winning this lottery, that is your chance of finding early treatable colon cancer is small, about .003 to .005.

What about the risks of the FOBT?

The false positive rate (the test says you have cancer or a polyp but you don’t) is high, about 45%.  This means that of 100 people who have a positive FOBT test, 45 will turn out not to have cancer or a pre-malignant polyp.

The false negative rate (you have colon cancer but the test says you don’t) ranges from 21% to 45% depending on which study you look at.  This means that of 100 people who have colon cancer, the test will be negative in 21-45 of them.

The over diagnosis rate is not applicable here, since any positive FOBT test will be followed by a colonoscopy.

Harm from the test itself is not a problem.  This is a non-invasive test that is done at home and there are no physical harms from the test.

What is the bottom line?

Although this test has a relatively high false positive and false negative rate, it is the only test that has been proven to decrease the death rate from colon cancer in people at average risk, it is non-invasive, safe and it is very inexpensive.  If you have a higher than average risk of dying from colon cancer, then colonoscopy is the better choice.