Purpose of a Colonoscopy

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A colonoscopy is used to help diagnose cancer and other gut-related abnormalities as well as to screen for cancers and remove them before they become a life-threatening condition. During a colonoscopy, a gastroenterologist uses a scope to detect (and often remove) pre-cancerous polyps and other lesions along the lining of the colon.

Scheduling a colonoscopy can be a drag because some fear that it is a painful procedure—medications are administered to promote relaxation and keep you comfortable during the procedure so that’s rarely the case—while others dread the much-maligned prep.

While having diarrhea, nausea, and feeling hungry may be experienced during the prep process, the discomfort is temporary. It can be consoling to know that it is endured in order to prevent cancer.

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

While a colonoscopy is used to 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 50 have a colonoscopy done to screen for colon cancer every ten years. Other visual or structural exams of the colon and rectum are CT colonography (virtual colonoscopy) and flexible sigmoidoscopy (FSIG). These procedures can be performed every five years.

Still, there are other less invasive screening tests called stool tests that can check your poop for signs of cancer. These include:

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

If any of these three tests yields an abnormal result, a colonoscopy is required to help diagnose the problem so 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 colorectal cancer
  • 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 (FAP) or Lynch syndrome (hereditary non-polyposis colon cancer or HNPCC)
  • 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 between the ages of 50 to 75, so it is recommended that this group get a colonoscopy every 10 years.

Those with inflammatory bowel disease such as Crohn’s disease and inflammatory bowel syndrome, a family history of colon cancer or precancerous polyps, and those with genetic conditions such as Lynch syndrome should receive a colonoscopy as rarely 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 also examine the rest of your body.

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

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

Over time, the blood loss can build up and can lead to low red blood cell counts or anemia. For this reason, blood is drawn from anyone in need of a colonoscopy to look for a low red blood cell count. The name of this test is a complete blood count (CBC).

Liver enzyme levels are also checked as colon cancer can spread to the liver. Tumor markers such as carcinoembryonic antigen (CEA) may also be elevated, although other conditions may elevate these levels, so they cannot be used alone to screen for or diagnose colon cancer.

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.

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  2. American Cancer Society. Colorectal cancer guideline: How often to have screening tests. Updated June 2019.

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