They test for colon (bowel) cancer mainly with stool tests and camera‑based exams of the bowel, often starting with a noninvasive stool test and doing a colonoscopy if anything looks abnormal. A colonoscopy is the key test because it lets doctors both find and remove polyps or take biopsies in the same procedure.

Main ways they test

  • Stool tests (at home or clinic)
    • Fecal immunochemical test (FIT): looks for tiny amounts of hidden blood in your stool once a year.
* Guaiac fecal occult blood test (gFOBT): older blood test for stool, affected more by diet and some medicines.

* FIT‑DNA / stool DNA tests (like Cologuard/Cologuard Plus, ColoSense): look for blood plus abnormal DNA or RNA from cancer or polyp cells in your stool, usually every 3 years if normal.
  • Scope tests (camera inside the bowel)
    • Colonoscopy: a flexible tube with a light and camera goes through the anus to see the entire colon and rectum; the doctor can remove polyps or take biopsies during the test.
* Sigmoidoscopy: similar camera test but only looks at the rectum and lower part of the colon (sigmoid colon); can also remove or biopsy growths there.
  • Imaging tests
    • CT colonography (“virtual colonoscopy”): a CT scan that creates images of the inside of the colon; no scope goes all the way through, but if it shows a polyp you still need a regular colonoscopy to remove it.

What actually happens in practice

  • For screening when you feel fine (typically starting around age 45 for average‑risk adults), doctors usually offer:
    • A yearly FIT or other stool test, with colonoscopy only if it comes back positive.
* Or a screening colonoscopy every 10 years if results are normal, depending on guidelines and your risk factors.
  • For symptoms (like rectal bleeding, long‑lasting bowel changes, unexplained iron‑deficiency anemia, or weight loss), doctors are more likely to go straight to a colonoscopy rather than only using stool tests.

What a colonoscopy feels like

  • Before the test:
    • You do a bowel prep the day before (laxative solution and a clear‑liquid diet) to flush the colon so the camera can see clearly.
  • During the test:
    • You lie on your side; the colonoscope is gently passed through the rectum into the colon.
    • You usually get sedation so you’re sleepy and comfortable, and many people remember little or nothing of the procedure.
* If the doctor sees polyps, they can be removed right away or biopsied; this is what actually prevents cancer from developing.
  • Afterward:
    • You may feel bloated or have mild cramping for a short time from the air used to open the colon.
* If biopsies or polyps were taken, results usually come back in several days.

How they confirm cancer

  • No matter which test finds something suspicious (stool test, CT colonography, sigmoidoscopy), the next step is usually a colonoscopy with biopsy.
  • A tiny sample of tissue is taken and examined under a microscope by a pathologist to confirm whether it is cancer, a benign polyp, or something else.

When to talk to a doctor

  • Talk to a doctor promptly if you notice:
    • Blood in or on your stool, or black/tarry stools.
* A change in bowel habits lasting more than a few weeks (diarrhea, constipation, thinner stools).
* Unexplained weight loss, fatigue, or iron‑deficiency anemia.
  • Even without symptoms, ask your doctor when you should start routine screening and which test makes the most sense for you, especially if you:
    • Are 45 or older.
    • Have a family history of colon or rectal cancer or certain polyps.
    • Have conditions like inflammatory bowel disease or certain inherited syndromes.

Bottom line: most people start with a stool test or a colonoscopy, and any abnormal result is followed up with a colonoscopy and biopsy to be sure.

Information gathered from public forums or data available on the internet and portrayed here.