How Does an At-Home Colorectal Cancer Screening Test Compare to a Colonoscopy? Answers From an Expert at Memorial Sloan Kettering Cancer Center

Dr. Robin Mendelsohn talking with a patient. s a gastroenterologist and an expert on colorectal cancer screening at Memorial Sloan Kettering Cancer Center.

Robin Mendelsohn is a gastroenterologist and an expert on colorectal cancer screening. She is Co-Director of MSK’s Center for Young Onset Colorectal and Gastrointestinal Cancer.

You probably know that at age 45, people at average risk are advised to begin screening for colorectal cancer. And you may be aware of various screening methods, particularly colonoscopy and the widely advertised at-home test called Cologuard®.

But many people have questions about colorectal cancer screening, including which test is right for them, what is “average risk,” and how their family’s medical history may affect their own health.

Robin Mendelsohn is a gastroenterologist at Memorial Sloan Kettering Cancer Center, and an expert on colorectal cancer screening. She is also Co-Director of the Center for Young Onset Colorectal and Gastrointestinal Cancer, the first center in the world devoted to the specific needs of people under 50 who face these cancers.

How important is it for people to be screened for colorectal cancer?

Colorectal cancer is the second leading cause of cancer deaths for men and women combined in the U.S. Over 52,000 deaths are expected from the disease this year.

However, overall death rate for these cancers — which mostly affect people in their 60s and older — have fallen by more than 50% since 1970.

Improved treatments and some changes in lifestyle have helped.

But research shows the biggest reason the death rate from colorectal cancer has dropped is that more people are getting screened. Screening means this disease can be caught earlier or prevented altogether.

Getting screened for colorectal cancer can save your life.

What are the screening tests for colorectal cancer?

There are a few major categories.

Stool-based tests examine a sample of a person’s stool to look for signs of cancer or growths inside the colon and rectum that can turn into cancer, called polyps. These tests include the multitarget stool DNA test that is known by most people by its trademark name, Cologuard, as well as other stool tests.

Endoscopic tests look directly inside the colon and rectum — this includes colonoscopy and flexible sigmoidoscopy.

Radiologic tests use the imaging technology called CT scans to make a visual model of the colon and search for polyps and cancers. This is sometimes called a virtual colonoscopy.

How do stool tests like Cologuard work, and what are the pros and cons?

There are a few types of stool tests.

  • Cologuard is called a multitarget stool DNA (MTsDNA) test.

Colorectal cancer and polyp cells shed DNA into the stool, as well as blood. This test detects blood in the stool as well as analyzing DNA for mutations and other genetic changes caused by polyps or colorectal cancer.


This test can be done at home, is noninvasive, and requires only one sample of a person’s stool. It is very good at detecting cancer, which is important because colorectal cancer is easier to treat when found early.


The MTsDNA test misses the majority of significant polyps, which are the lesions that can grow into cancer. Further, if a Cologuard test does come back positive, a colonoscopy is then needed to evaluate for polyps or cancer. Also, this test has a false positive rate of 10% to 12%. Medicare and some insurers will cover one test every three years. But this kind of screening is more expensive than some other stool tests.

  • Fecal immunochemical test (FIT)

This test detects blood in the stool, which can indicate polyps or colorectal cancer


This test requires only one sample, is noninvasive, doesn’t require a colon preparation, and is relatively inexpensive.


This test misses the majority of significant polyps and must be followed with a colonoscopy if it comes back positive. Also, the test may detect blood that is not coming from the colon or rectum, so it can be more difficult to pinpoint the source of concern. It is recommended that people use this method each year.

  • Guaiac-based fecal occult blood test (gFOBT)

This test also detects blood in the stool.


It is noninvasive and the cheapest of the screening methods.


This test is not good at detecting polyps and is more likely to result in false positives than other methods. It must be done yearly.

How do colonoscopies work, and what are the pros and cons?

We consider colonoscopies the gold standard of colorectal cancer screening. A colonoscope is a thin, flexible tube inserted into the colon. The tube contains a camera to look for abnormal growths that can grow into cancer, called polyps. There is also a channel that the doctor can put tools through to take out polyps.


A colonoscopy can not only find polyps before they become cancer but also prevent cancer from occurring by snipping out the polyps. It’s the only screening method that can remove polyps.

A colonoscopy detects almost all significant polyps and cancers, so it’s very accurate. If nothing is found, a person generally does not need another colonoscopy for 10 years.


Sedation is usually required. Because it is an invasive procedure, there are risks, although they are very small. A colonoscopy requires consuming preparations to flush out the colon, which some people may consider unpleasant. Colonoscopy has been shown to be cost-effective but is significantly more expensive than other screening methods. Colonoscopy is covered by most insurance plans.

The best test for colorectal screening is the one that gets done and done well.
Robin Mendelsohn Co-Director, Center for Young Onset Colorectal and Gastrointestinal Cancers

What is flexible sigmoidoscopy?

This is similar to a colonoscopy but uses a shorter tube that only examines one side of the colon.


It can both detect and remove polyps. It usually doesn’t require a full preparation, as with a colonoscopy, and can be done by qualified medical personnel other than a doctor without anesthesia.


Because a flexible sigmoidoscopy only examines part of the colon, some people have said it’s like taking a mammogram of only one breast — helpful, but not complete. If a polyp is found, a full colonoscopy should be performed because there is a greater risk of polyps or cancer in the rest of the colon. The procedure is invasive, so there are some risks. It is less expensive than a colonoscopy but does cost more than other screening methods.

What about a CT colonoscopy (CTC)?

This method uses CT scans to construct images of the colon and rectum that can be interpreted by a doctor.


This method is good at detecting both polyps and cancer.


A colonoscopy is needed if anything is found. This method can also miss growths called “flat lesions” and requires an experienced radiologist, who may not be available everywhere. There is a small risk from radiation exposure, as well as findings from outside the colon that require further workup.

What is the best test?

The best test for colorectal screening is the one that gets done and done well.

At MSK, colonoscopy is the preferred method because it can detect polyps and cancer and actually prevent cancer by snipping out polyps before they can grow into cancer.

However, if you’re not willing to undergo a colonoscopy, another method may be the best test for you. But if any of these other tests are positive, you will need to have a colonoscopy.

Learn more about screening tests for colorectal cancer

What does it mean to be at “average risk” of colorectal cancer?

There are four questions:

  • Do you have a family or personal history of colorectal polyps or cancer?
  • Do you have inflammatory bowel disease like ulcerative colitis or Crohn’s disease?
  • Do you have a genetic predisposition such as Lynch syndrome?
  • Have you had radiation to the abdomen or pelvis?

If the answers to all these questions is no, you are at average risk.

Learn more about the guidelines for colorectal screening

What does “family history” mean?

That a first-degree relative had polyps or colorectal cancer — meaning parents, siblings, or children.

Sometimes people say, “My great-uncle had polyps.” That doesn’t count.

In 2021, an expert task force lowered the age to begin screening. Why?

The recommended age to begin screening for people with average risk was lowered from 50 years of age to 45. That’s because there has been a troubling rise in colorectal cancer among people younger than 50, for reasons we don’t yet understand. This is happening even as the rate of colorectal cancer is falling among older people — again, mostly because of effective screening.

To help these younger patients, MSK established the first center in the world devoted to the specific needs of people under 50, the Center for Young Onset Colorectal and Gastrointestinal Cancer.

If a person of any age is experiencing symptoms that trouble them for more than a few days—particularly rectal bleeding or blood in their stool — they should talk to a healthcare provider.

Colorectal cancer is most common in older people, but no one is “too young” to develop it.