Purpose of a Colonoscopy

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A colonoscopy is primarily used to help identify colon polyps and remove them so that they do not turn into colon cancer. The test can also identify other gut-related abnormalities and diagnose colon cancer.

Colonoscopy looks for lesions such as polyps and can remove them to prevent cancer
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Diagnoses Related to Colonoscopy

While a colonoscopy is used most frequently to look for precancerous lesions, it is also often used to diagnose other gut-related issues or causes of abdominal pain, chronic constipation, chronic diarrhea, rectal bleeding (such as hemorrhoids), colitis (inflammation of the colon), and diverticular disease (outpouching of colon's wall).

Cancer Screening

It is recommended that all adults of average risk over the age of 45 have a colonoscopy done to screen for colon cancer every 10 years, or more frequently if necessary. Other visual or structural exams of the colon and rectum are CT colonography (virtual colonoscopy) and flexible sigmoidoscopy. These procedures can be performed every five years.

Still, there are other, less-invasive screening tests called stool tests that can check for signs of cancer. These include the following.

  • Fecal immunochemical test: A yearly at-home test that looks for occult or hidden blood in your stool.
  • Guaiac-based fecal occult blood test: A yearly at-home test that detects occult blood in your stool via a chemical reaction.
  • Stool DNA test: Looks for certain abnormal sections of DNA from cancer or polyp cells. This test can be done every three years.

Currently, Cologuard is the only stool DNA test available in the U.S., and it tests for both DNA changes and blood in the stool.

If any of these three tests yield an abnormal result, a colonoscopy is required to further evaluate the findings. As a result, many healthcare professionals encourage colonoscopy as the primary screening tool for colon cancer.

Screening with a colonoscopy is important, because symptoms of colon cancer may not show up right away. Some of the symptoms to look out for are:

  • A change in bowel habits, such as diarrhea, constipation, or narrowing of the stool, that lasts for more than a few days
  • A feeling that you need to have a bowel movement that's not relieved by having one
  • Rectal bleeding with bright red blood
  • Blood in the stool, which might make the stool look dark brown or black
  • Cramping or abdominal (belly) pain
  • Weakness and fatigue
  • Unintended weight loss

One or more of these symptoms may appear at the same time. If they do, please contact a healthcare professional and consider scheduling a colonoscopy even if you are not due for one, as your condition may have changed since your last colonoscopy.


For screening, people are considered to be at average risk if they do not have:

  • A personal history of colorectal cancer or certain types of polyps
  • A family history of certain types of colon polyps
  • A personal history of inflammatory bowel disease (ulcerative colitis or Crohn’s disease)
  • A confirmed or suspected hereditary colorectal cancer syndrome, such as familial adenomatous polyposis or Lynch syndrome (hereditary non-polyposis colon cancer)
  • A personal history of getting radiation to the abdomen (belly) or pelvic area to treat a prior cancer

Anyone who is experiencing the aforementioned symptoms of colon cancer is eligible for a colonoscopy, regardless of age. In fact, colon cancer is on the rise in young adults, so colon health is on every health professional’s radar.

Still, older adults are at highest risk, especially those between the ages of 45 to 75, so it is recommended that this group get a colonoscopy every 10 years or more often, depending on your family history or previous colonoscopy results.

Those with inflammatory bowel disease such as Crohn’s disease or ulcerative colitis, a family history of colon cancer or precancerous polyps, and those with genetic conditions such as Lynch syndrome should receive a colonoscopy as early as 40 years old.

According to the American Cancer Society, for people ages 76 through 85, the decision to be screened should be based on a person’s preferences, life expectancy, overall health, and prior screening history. People over 85 should no longer get colorectal cancer screening.

Tests and Labs

The first step in diagnosing any gut-related issue is to take a complete medical history, including a family history, and perform a physical exam. As part of a physical exam, a healthcare provider will feel your abdomen for masses or enlarged organs and examine the rest of your body.

You may also have a digital rectal exam. During this test, the doctor inserts a lubricated, gloved finger into your rectum to feel for any abnormal areas and also test for blood.

Symptoms of colon cancer are non-specific. Sometimes blood can be easily seen in the stool, or make the stool look darker, but often the stool looks normal.

Over time, blood loss can lead to low red blood cell counts or anemia. For this reason, blood may be drawn from anyone in need of a colonoscopy to look for a low red blood cell count. This test is called a complete blood count.

If a suspected colorectal cancer is found by any screening or diagnostic test, a small piece of tissue is removed (usually during colonoscopy) and sent to a lab, where it is looked at more closely to confirm the diagnosis.

2 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. American Cancer Society. Testing for colorectal cancer: How is colorectal cancer diagnosed?

  2. American Cancer Society. Colorectal cancer guideline: How often to have screening tests.

By Shamard Charles, MD, MPH
Shamard Charles, MD, MPH is a public health physician and journalist. He has held positions with major news networks like NBC reporting on health policy, public health initiatives, diversity in medicine, and new developments in health care research and medical treatments.