More than 41 million Americans need colon cancer screening (search), a CDC study shows.

Colon cancer (search) is the second most deadly cancer in the U.S. Yet it's largely preventable. Screening tests can find nearly all colon cancers before they become dangerous. Of course, a person has to show up for these tests. And more than 41 million Americans don't.

"This is the first time that the size of the unscreened population has been measured, then compared to the number of tests being performed," CDC researcher Laura C. Seeff, MD, says in a news release.

Seeff's studies — one estimating the number of people who need colon cancer screening, and another estimating how much capacity the U.S. health care system has for colon cancer screening — appear in the Dec. 1 issue of Gastroenterology.

Seeff and colleagues focused on people at low risk of colon cancer. Beginning at age 50, everybody should be screened for the disease. There are four different tests:

—The FOBT is a test for blood in the stool. FOBT screening (search) is needed once a year.

Double-contrast barium enemas (search) are needed once every five years.

—Sigmoidoscopy uses an endoscope to view the lower portion of the colon. This test is needed once every five years.

—Colonoscopy uses an endoscope to view the entire colon. Colonoscopy requires sedation and is usually done by a gastroenterologist. This test is needed once every 10 years.

Any of these tests can prevent colon cancer. Some experts argue that colonoscopy is best, for two reasons. First, colonoscopy examines the entire colon and may find polyps missed by sigmoidoscopy. Second, any other test that suggests trouble means a colonoscopy is needed. Starting with colonoscopy eliminates the need for two procedures.

But there's a catch. Seeff and colleagues find that it would take 10 years to give a colonoscopy to every one of the 41 million Americans who needs colon cancer screening. On the other hand, FOBT screening on all 41 million, plus follow-up colonoscopies for those who test positive, would take just one year.

This means that doctors should stop recommending screening colonoscopies for patients under the age of 60, argues gastroenterologist Theodore R. Levin, MD. Levin is regional director of colorectal cancer screening for Kaiser Permanente, Northern California. His editorial accompanies the Seeff studies.

Levin argues that those in their 50s should forgo colonoscopy in favor of sigmoidoscopy.

"Shifting colonoscopy resources away from excessive, unnecessary surveillance examinations could increase the available supply of screening colonoscopy," Levin says in a news release.

By Daniel J. DeNoon, reviewed Brunilda Nazario, MD

SOURCES: Seeff, L.C. Gastroenterology, Dec. 1, 2004; vol 127: pp 1670-1677 and 1661-1669. Levin, T.R. Gastroenterology, Dec. 1, 2004; vol. 127: pp 1841-1844.