What Is the Best Way To Test for Colorectal Cancer?

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The summary below is from the full report titled "Accuracy of Screening for Fecal Occult Blood on a Single Stool Sample Obtained by Digital Rectal Examination: A Comparison with Recommended Sampling Practice." It is in the 18 January 2005 issue of Annals of Internal Medicine (volume 142, pages 81-85). The authors are J.F. Collins, D.A. Lieberman, T.E. Durbin, D.G. Weiss, and the Veterans Affairs Cooperative Study #380 Group.

What is the problem and what is known about it so far?

Cancer of the large intestine (colorectal cancer) is one of the most common causes of cancer death. Early treatment, before cancer has spread to other organs, often cures colorectal cancer. Colorectal cancer begins as a colon polyp, a small mushroom-like growth on the inside surface of the colon. The cells in a colon polyp can't spread to other organs, but some types of colon polyps contain cells that will develop the ability to spread as the polyp grows.

The goal of screening is to detect cancer before it has spread or when it is still a polyp. Some cancers and polyps bleed, although usually the blood is invisible to the naked eye. The simplest, least costly way to screen for colorectal cancer is to test for blood in the stool. If blood is present, the physician should examine the entire colon (colonoscopy) for polyps or cancer.

Why did the researchers do this particular study?

To see how often patients with cancer or polyps have blood in the stool. The authors compared 2 methods of getting stool for testing. One method uses a single sample obtained when the doctor does a rectal examination; the other method uses 6 samples obtained by the patient at home.

Who was studied?

2665 patients at 13 Veterans Affairs medical centers. None had symptoms of colorectal cancer.

How was the study done?

All patients had a test for stool blood after a rectal examination. All patients obtained 6 samples of stool at home and returned them to the doctor for testing, and all patients had a complete examination of the colon (colonoscopy).

What did the researchers find?

Two hundred eighty-four patients had cancer or large polyps that were likely to become cancer. Only 4.9% of these patients had positive results on a test for blood in the single sample of stool obtained during the rectal examination. Many more of the 284 patients (23.9%) had positive results on the 6 samples obtained at home. Therefore, the 6-sample method is a much better way to obtain stool for testing than the method of using a single sample obtained during a rectal examination.

What were the limitations of the study?

Colonoscopy may have failed to detect colorectal cancer or large polyps in a few patients.

What are the implications of the study?

Testing for blood in a sample of stool obtained after a rectal examination in the doctor's office is a very poor way to detect colorectal cancer or large polyps. Physicians should not use it as a screening test. Instead, they should use the recommended method for obtaining stool for testing, which is to ask the patient to obtain 6 samples at home.