A colonoscopy gives your doctor a real-time, high-definition view of the entire lining of your large intestine, from the rectum to where the small intestine connects. It can reveal polyps, cancerous growths, inflammation, bleeding sources, structural changes like diverticula, and hemorrhoids. Most people getting a screening colonoscopy will have at least one finding: current benchmarks expect doctors to find precancerous polyps in at least 35% of all screening patients.
Polyps and Precancerous Growths
Polyps are the most common and most important finding. These are small growths on the inner lining of the colon that range from a few millimeters to several centimeters across. Most are harmless at the time they’re found, but certain types can slowly develop into cancer over years, which is why doctors remove them on the spot during the procedure.
Not all polyps carry the same risk. The three main types differ significantly:
- Adenomatous polyps are the classic precancerous type and account for the large majority of polyps found. They gradually develop abnormal cell changes over time, and those that progress far enough will become malignant. Tubular adenomas make up over 80% of this category, with villous and tubulovillous types making up the rest.
- Sessile serrated polyps tend to appear in the right side of the colon and are harder to spot because they sit flat against the wall. Without abnormal cell changes they have low cancer risk, but when those changes are present, the risk becomes significant.
- Hyperplastic polyps are the most common and least concerning. They cluster in the lower colon and have very low malignant potential.
When a polyp is removed, it goes to a pathology lab. Your results typically come back within one to two weeks and will describe the polyp type, size, and whether any abnormal cells were found. That pathology report determines your follow-up schedule, which is why two people who both “had a polyp removed” can end up with very different recommendations for their next colonoscopy.
Signs of Colorectal Cancer
Colonoscopy remains the gold standard for detecting colorectal cancer because the doctor can both see suspicious tissue and take a biopsy in the same session. Cancerous areas often appear as irregular, swollen, or ulcerated patches on the colon wall. Some cancers grow as large masses that partially block the colon, while others are flat or slightly depressed lesions that are subtler to identify.
Advanced imaging techniques help distinguish suspicious growths from harmless ones. Using narrow-band imaging, which highlights blood vessel patterns on the surface, doctors look for brownish discoloration, irregular surface textures, and specific color patterns at the edges of a lesion. Flat or depressed lesions with these features are more likely to harbor high-grade abnormal cells or early invasive cancer.
Colorectal cancer is the most common cause of large bowel obstruction, responsible for about 60% of cases. When a tumor grows large enough to narrow the colon significantly, the colonoscopy can identify both the blockage and the underlying cause.
Inflammation and Inflammatory Bowel Disease
Redness, swelling, and sores on the colon lining all point to inflammation, but the pattern of that inflammation tells a very different story depending on the condition.
Ulcerative colitis produces continuous inflammation that starts at the rectum and extends upward without gaps. The doctor sees a loss of the normal blood vessel pattern on the surface, a grainy-looking lining, and erosions. Crohn’s disease looks quite different: it causes patchy, segmented inflammation with stretches of healthy tissue in between. The hallmarks include deep ulcers, a cobblestone-like texture, and sometimes involvement of the area around the anus while the rectum itself looks normal.
Some inflammation doesn’t fit neatly into a chronic category. Your report might note “nonspecific inflammation,” meaning mild redness or irritation without a clear cause, which could stem from a recent infection or even the bowel prep itself. In other cases, the colon lining looks completely normal during the procedure, but biopsies reveal microscopic colitis, a condition that only shows up under a microscope. This is one reason doctors sometimes take tissue samples even when everything looks fine visually.
Diverticula and Hemorrhoids
These two findings are extremely common and, in most cases, completely incidental. Between 40% and 60% of people have diverticula, small pouches that form in the muscular wall of the colon and bulge outward. They cluster most often in the sigmoid colon, just above the rectum, and become more common with age. Diverticula only cause problems if one bleeds or becomes infected (diverticulitis). Finding them on a colonoscopy typically requires no treatment at all.
Internal hemorrhoids, the kind inside the lower rectum, are also a routine finding. Everyone has the cushion-like clusters of veins they develop from, and they only become noteworthy when they swell enough to bleed or prolapse. If you’ve noticed bright red blood in the toilet, hemorrhoids are one of the first things a colonoscopy can confirm or rule out. External hemorrhoids, which sit outside the anus, are more likely to cause itching or pain but are usually identified during a physical exam rather than the colonoscopy itself.
Bleeding Sources
One of the most valuable uses of colonoscopy is pinpointing the source of unexplained lower GI bleeding. The camera can identify a wide range of causes: bleeding diverticula, fragile or abnormal blood vessels (called angiodysplasia), ulcers, polyps, tumors, and areas of inflammation from conditions like ischemic colitis or IBD. In many cases, the doctor can treat the bleeding during the same procedure using clips, cauterization, or injection.
What a Colonoscopy Can Miss
Colonoscopy is highly effective, but it isn’t perfect. Flat and sessile polyps are missed at a rate of about 33%, compared to roughly 8% for polyps that grow on a stalk. Size matters too: polyps 5 mm or smaller are missed about 35% of the time, while those 10 mm or larger are missed less than 5% of the time. When multiple polyps are present, the chance of overlooking one goes up substantially. In patients with five or more polyps, the odds of a missed polyp are more than 11 times higher than in patients with a single polyp.
Bowel preparation quality plays a major role in what the doctor can see. Each section of the colon is scored on how clean the lining is, on a scale from 0 (solid stool blocking the view) to 3 (perfectly clean). When the total prep score is adequate (5 or higher out of 9), the polyp detection rate reaches 40%. When prep is poor, detection drops to 24%, and about 73% of those patients are asked to come back and repeat the procedure. Following your prep instructions closely is one of the most impactful things you can do for the accuracy of your results.
Understanding Your Results
After the procedure, you’ll typically get preliminary findings the same day from your doctor, covering what was seen and whether anything was removed or biopsied. If polyps were removed or tissue was sampled, the full pathology results arrive within one to two weeks.
Your report will describe the extent of the exam (how far the camera reached), the quality of the bowel prep, and any findings organized by location in the colon. Common terms you might see include “tubular adenoma” for the most typical precancerous polyp, “hyperplastic polyp” for a benign growth, “chronic inflammation” for signs of a long-term condition, or “diverticulosis” for those common pouches. The pathology results are what ultimately determine whether a polyp was precancerous, and they drive the recommended interval before your next screening, which can range from one year to ten years depending on what was found.
For average-risk adults, the U.S. Preventive Services Task Force recommends screening starting at age 45, with repeat colonoscopies every 10 years if no significant findings are present. Any polyps, inflammation, or family history can shorten that interval considerably.