April 12, 2022

How Accurate Is a Stool Test for Colon Cancer Screening?

The effectiveness of at-home screening options

An illustration of a gloved hand putting a lid on a stool sample

Can a colon cancer screening really be as simple as mailing a sample of your stool (poop) to a lab for analysis? The answer might be yes — but don’t cancel that colonoscopy just yet.

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At-home stool tests can be an effective tool for detecting colon cancer. But they’re not as accurate as a colonoscopy and they don’t offer a way of finding and removing polyps that could turn cancerous.

So, are stool tests even worth trying? Colorectal surgeon Arielle Kanters, MD, explains the benefits and shortcomings.

The importance of colon cancer screenings

Screening for colon cancer is vital because symptoms often don’t appear in the disease’s early and more treatable stages. “It’s the best way to identify issues ahead of any problems,” says Dr. Kanters.

Colorectal cancers typically begin as growths called polyps, which form on the inner lining of your colon or rectum (anus). Finding and removing polyps before they become cancerous is the most effective way of preventing colon cancer.

Early detection of colorectal cancer — before it has a chance to spread outside of your colon or rectum — increases your odds of successful treatment, according to the American Cancer Society.

The five-year survival rate for colorectal cancer is about 90% when the disease is found early. Here’s the concern, though: Only about 4 of 10 colorectal cancers are detected at this stage.

Types of stool tests

Stool tests are promoted as easy, at-home ways to check for potential signs of colorectal cancer. But it’s important to note their limitations in detecting precancerous polyps compared to a colonoscopy.

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“Relying on these fecal tests means that you’re potentially missing out on an early pre-cancer that could be eliminated and never turn into something scarier,” explains Dr. Kanters.

There are three primary stool tests offered. Each asks you to collect a stool sample during a bathroom visit and send it to a lab for analysis.

Fecal immunochemical test (FIT)

The fecal immunochemical test (FIT) looks for hidden blood in your stool that could be coming from colon cancer. Studies show that FITs can be nearly 80% accurate in detecting colorectal cancer. The test should be done annually.

Guaiac-based fecal occult blood test (gFOBT)

This annual test is similar to the FIT in that it checks for the presence of blood. It does require dietary changes before collecting a sample, says Dr. Kanters. Research shows that gFOBTs are about 71% accurate in detecting colorectal cancer.

Stool DNA testing

This test examines DNA in your stool for evidence of cells shed by colorectal cancer. The DNA test is 92% effective at detecting cancers. (DNA tests, however, do return more false positives than FITs.) This test should be done once every three years.

What happens if a stool test is positive?

If a FIT, gFOBT or stool DNA test comes back positive, you should schedule a colonoscopy with your healthcare provider to find the underlying cause of the positive test. A colonoscopy also allows your doctor to find and remove polyps.

(Watch as Dr. Kanters explains different colorectal cancer screening options in this video.)

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Is a stool test as good as a colonoscopy?

While stool tests can be effective, a colonoscopy remains the gold standard for cancer screening, stresses Dr. Kanters. In one single procedure, it allows for the detection and removal of polyps to help prevent cancer.

“In my mind, a colonoscopy is always the way to go,” she says.

A colonoscopy typically takes 30 minutes to an hour and is done on an outpatient basis. It’s extremely safe and effective, and the preparation and process aren’t nearly as bad as many might fear.

But if you can’t get a colonoscopy — perhaps because of access or insurance — a stool test is better than doing no screening at all.

“Whatever we can do to improve our screening practices is fantastic,” says Dr. Kanters. “These tests can provide information that can help you seek out additional testing so you know what you’re dealing with.”

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