A colonoscopy looks for polyps, cancer, sources of bleeding, signs of inflammatory bowel disease, and other abnormalities along the entire length of the colon and rectum. Its primary purpose is detecting and removing precancerous growths before they turn into colorectal cancer, a process that cuts cancer incidence by roughly 52% and cancer mortality by 62%. Screening is now recommended starting at age 45 for people at average risk.
Polyps: The Main Target
Polyps are small growths on the inner lining of the colon. Most are harmless, but certain types can slowly develop into cancer over a period of years. The central job of a colonoscopy is to find these growths and remove them before that transformation happens. Up to 80% of sporadic colorectal cancers arise through a well-understood sequence in which a normal cell becomes an adenoma (a precancerous polyp) and eventually progresses to cancer.
Not all polyps carry the same risk. The ones your doctor is most concerned about include:
- Tubular adenomas: The most common precancerous polyp. They generally carry the lowest risk among adenomas, and most show only mild cell changes (low-grade dysplasia). They’re typically removed on the spot.
- Sessile serrated lesions: Flat or slightly raised growths that tend to appear in the upper colon. They can be hard to spot because they blend in with surrounding tissue. Most are benign, but larger ones or those with abnormal cell changes have real potential to become cancer.
- Traditional serrated adenomas: Less common, but they contain dysplasia (precancerous cell changes) and carry clear malignant potential.
- Hyperplastic polyps: These lack dysplasia and have minimal cancer risk. Small ones found in the lower colon are generally left alone or noted but not a cause for concern.
When the doctor spots a polyp, what happens next depends on its size. Polyps smaller than about 6 millimeters can often be removed with biopsy forceps during the exam itself. Larger polyps, roughly 6 to 9 millimeters or bigger, are typically removed with a snare, a thin wire loop that cuts the polyp from the colon wall. Either way, the tissue is sent to a lab so a pathologist can determine exactly what type of polyp it was and whether the cells showed precancerous changes.
What Your Pathology Report Means
After polyps are removed, the pathology report uses specific terms that describe how abnormal the cells looked under a microscope. Low-grade dysplasia means the cells show early, mild precancerous changes. The risk of cancer is low, and standard follow-up intervals apply. High-grade dysplasia is more serious: the cells look significantly more atypical, the risk of progressing to cancer is higher, and your doctor will recommend an earlier repeat colonoscopy and closer surveillance going forward.
If no polyps are found and the exam is otherwise normal, most people won’t need another colonoscopy for 10 years. Finding one or two small tubular adenomas with low-grade dysplasia typically means a repeat in 7 to 10 years. More polyps, larger polyps, or high-grade dysplasia shorten that interval considerably.
Colorectal Cancer
Beyond precancerous polyps, a colonoscopy can identify cancer that has already developed. Cancerous growths often appear as irregular masses, areas of ulceration, or segments of the colon wall that look clearly abnormal. The doctor will take biopsies from any suspicious area. Catching cancer at an early stage, before it has spread beyond the colon wall, dramatically improves survival. That is the entire logic behind routine screening: find it early or, better yet, find and remove the polyp before it ever becomes cancer.
Sources of Bleeding
If you’ve noticed blood in your stool or have unexplained anemia, a colonoscopy can locate exactly where the bleeding is coming from. Diverticular bleeding, where a blood vessel inside a small pouch in the colon wall ruptures, is the most common cause of lower gastrointestinal bleeding, responsible for roughly 40% of episodes. During the exam, the doctor looks for active bleeding, visible blood vessels within diverticula, or clots stuck to the colon wall that indicate a recent bleed.
Other bleeding sources a colonoscopy can identify include hemorrhoids (internal ones that aren’t visible from outside), vascular malformations where fragile blood vessels form on the colon lining, and areas of ischemic colitis where reduced blood flow has damaged the tissue.
Inflammatory Bowel Disease
A colonoscopy is one of the primary tools for diagnosing and monitoring ulcerative colitis and Crohn’s disease. Each condition leaves distinct visual signatures on the colon lining.
In ulcerative colitis, the doctor typically sees swollen, reddened mucosa with a loss of the normal blood vessel pattern visible through healthy tissue. The inflammation is continuous, starting at the rectum and extending upward without gaps. In more severe cases, the lining develops erosions, shallow ulcers, and areas that bleed spontaneously when touched by the scope.
Crohn’s disease looks different. The inflammation tends to be patchy, with diseased segments separated by stretches of normal-looking tissue. Deep ulcers arranged in a lengthwise pattern are characteristic, and when they become extensive, the surrounding swollen tissue creates a cobblestone appearance. The doctor also looks for strictures (narrowed segments where chronic inflammation has scarred and thickened the wall) and signs of fistulas, abnormal tunnels between the colon and adjacent structures. The rectum is often spared in Crohn’s, which helps distinguish it from ulcerative colitis.
During these exams, biopsies are taken from both inflamed and normal-appearing areas. The microscopic pattern of inflammation helps confirm the diagnosis and guide treatment decisions.
How Prep Quality Affects the Exam
Everything a colonoscopy looks for depends on the doctor being able to actually see the colon lining. That’s why bowel preparation matters so much. Doctors score how clean the colon is using a standardized system that rates three sections of the colon (right, middle, and left) on a scale from 0 to 3. A score of 0 means solid stool is blocking the view entirely. A score of 3 means the lining is perfectly visible with no residual debris. The three segment scores are added together for a total between 0 and 9.
If prep quality is poor, flat polyps like sessile serrated lesions can hide under residual stool, and subtle signs of inflammation can be missed entirely. A colonoscopy done with inadequate prep may need to be repeated sooner than usual, sometimes within a year, essentially negating the effort you put into the first one. Current quality benchmarks expect doctors to find at least one adenoma in 35% or more of screening exams. Falling below that threshold suggests polyps are being missed.
Other Findings
A colonoscopy can also detect diverticulosis (the pouches themselves, separate from any bleeding), which is extremely common in adults over 50 and usually requires no treatment. It can identify colonic strictures from causes other than Crohn’s, including prior radiation therapy or scarring from surgery. Rarely, the exam turns up unexpected findings like parasitic infections, melanosis coli from chronic laxative use, or benign lipomas (fatty lumps) under the colon lining.
For people with a history of colorectal cancer who have had surgery, surveillance colonoscopies look specifically at the surgical site for any signs of recurrence and scan the remaining colon for new polyps, since having had cancer once increases the risk of developing new growths.