Colonoscopy

What is colonoscopy?

A colonoscopy is an examination of the whole large bowel with a thin, flexible tube carrying a camera. It is used to investigate symptoms, screen for bowel cancer and remove polyps. It is usually done under sedation and takes about 20 to 40 minutes.

During colonoscopy, a flexible tube with a camera is passed through the back passage to examine the lining of the rectum and colon. Air is used to open the bowel so the lining can be seen clearly. If polyps or abnormal tissue are found, they can be removed or sampled without a separate operation. Allow extra time before and after for preparation and recovery from sedation.

Who should have a colonoscopy?

A colonoscopy is advised for screening from age 50, and earlier with a family history of bowel cancer or polyps. It is also advised at any age for rectal bleeding, a lasting change in bowel habit, unexplained anaemia, or follow-up after polyps.

• Routine colorectal cancer screening
• Bleeding from the bowel or blood in the stool
• Persistent diarrhoea, constipation, or abdominal pain
• Unexplained anaemia
• Follow-up after polyps or cancer
• A family history requiring surveillance

When should I seek help after a colonoscopy?

Most people feel well after a colonoscopy, with some bloating that settles quickly. Contact the clinic or go to an emergency department for severe abdominal pain, heavy bleeding, fever, a swollen or rigid abdomen, or dizziness or fainting.

The warning signs below need prompt attention.

• Severe abdominal pain after the procedure
• Heavy bleeding or fever
• A swollen or rigid abdomen
• Dizziness or fainting

What can a colonoscopy find?

A colonoscopy shows the lining of the whole colon and rectum directly. It can find polyps, cancer, inflammation, diverticula and sources of bleeding. Polyps can be removed and tissue sampled during the test, so many problems are found and treated in a single visit.

• Direct visual inspection of the bowel lining
• Removal of polyps (polypectomy)
• Biopsy of abnormal areas
• Photographs and a report of findings

How do I prepare for a colonoscopy?

Preparation clears the bowel so it can be seen properly. You follow a restricted diet for about a day and drink a bowel-cleansing solution, with exact instructions from the clinic. Tell Dr Wong about blood thinners, diabetes medicines and iron tablets before the test.

• Follow the diet and timing in your written instructions
• Finish the whole bowel-cleansing drink, because clear views depend on it
• Do not stop or change regular medicines unless the clinic tells you to
• Arrange for an adult to take you home and stay with you for a while
• For 24 hours after sedation, do not drive, operate machinery, drink alcohol or make important decisions

What is the difference between a screening and a diagnostic colonoscopy?

A screening colonoscopy is done when you have no symptoms, to find and remove polyps or catch cancer early. A diagnostic colonoscopy is done to explain a symptom or an abnormal test, such as bleeding, a change in bowel habit, anaemia or a positive stool test. The procedure is the same.

The difference matters mainly for timing and follow-up. A screening test can be planned at a convenient time, while symptoms such as ongoing bleeding or weight loss should be investigated without long delay. After either kind of test, the findings decide when, or whether, you need another one.

• Screening: from age 50 for people at average risk in Singapore, repeated about every 10 years if the bowel is normal and the examination is of good quality
• Earlier screening: for people with a close relative diagnosed with bowel cancer at 60 or younger, or two or more close relatives affected, colonoscopy is advised from age 40 or 10 years before the youngest case in the family, whichever is earlier
• Diagnostic: for rectal bleeding, a lasting change in bowel habit, unexplained anaemia, abdominal pain with other warning signs, or a positive faecal immunochemical test
• Surveillance: planned follow-up after polyps or bowel cancer have been removed, at intervals set by what was found

Should I do a FIT kit or go straight to colonoscopy?

Both are accepted ways to screen for bowel cancer in Singapore. A faecal immunochemical test (FIT) checks a stool sample for hidden blood and is repeated every year from age 50. Colonoscopy looks directly at the bowel and, if normal, is usually repeated after 10 years. A positive FIT needs a colonoscopy.

FIT detects human blood from the bowel and needs no special diet. It is simple and free at participating clinics under the national screening programme, but a single test can miss a cancer that is not bleeding at the time, which is why it must be repeated every year to work well. Colonoscopy is more invasive and needs bowel preparation and usually sedation, but it examines the whole bowel and can remove polyps in the same sitting.

What matters most is choosing a test you will complete. If you have a family history of bowel cancer, a hereditary condition, inflammatory bowel disease or symptoms, a stool test is not the right choice and colonoscopy is advised instead.

• FIT: home stool sample, no diet change, repeated yearly, a positive result leads to colonoscopy
• Colonoscopy: direct view of the whole large bowel, polyps removed at the same time, repeated about every 10 years if normal
• Colonoscopy carries small risks, including bleeding and, rarely, a tear in the bowel wall
• Symptoms or a strong family history call for colonoscopy rather than a stool test

Will I be awake, and what sedation options are there?

Most colonoscopies are done with sedation given through a small tube in a vein in your arm or hand, together with pain relief, so you are relaxed and comfortable. Some people need deeper anaesthesia, and some choose lighter sedation. After any sedation you cannot drive for 24 hours and need someone to take you home.

Depending on the medicines used, you may be drowsy but able to respond, or unaware of the test altogether, and many people remember little of it. The level is chosen with you, taking into account your health, any previous experience of colonoscopy and your preference. Tell the team about heart or lung problems and any past reactions to sedatives.

• Sedation with pain relief through a drip is the usual approach
• Some patients have deeper anaesthesia given by an anaesthetist
• In some centres, inhaled gas and air is offered as an alternative, though not every hospital provides it
• Expect to rest under observation while the sedation wears off before you go home
• For 24 hours after sedation, do not drive, operate machinery, drink alcohol or make important decisions

Which medicines should I discuss before a colonoscopy?

Tell the team about every medicine and supplement you take, well before the test. Blood thinners, diabetes medicines, iron tablets and weight-loss or diabetes injections such as semaglutide may need a plan. Do not stop a blood thinner on your own, because the risk of a clot or stroke can outweigh the bleeding risk.

British and European guidance sorts colonoscopy into lower-risk procedures, such as a diagnostic look with biopsies, and higher-risk procedures, such as removing larger polyps. For lower-risk procedures, aspirin and often warfarin can usually continue, with a blood test to check warfarin levels. For higher-risk procedures, some blood thinners are paused for a set number of days, and people at high risk of clots may need a temporary injection instead.

GLP-1 receptor agonists, used for diabetes and weight loss, slow the emptying of the stomach. Multi-society guidance published in 2024 says most people can continue them. Those on higher doses or with stomach side effects may be asked to follow a liquid diet for 24 hours beforehand, and a planned procedure may be timed after the early dose-increasing weeks, to lower the risk of food remaining in the stomach under sedation.

• Aspirin taken for heart or stroke protection is not usually stopped
• Clopidogrel, ticagrelor, warfarin and newer anticoagulants such as apixaban, rivaroxaban, edoxaban or dabigatran need an individual plan, sometimes with your cardiologist
• Patients with coronary stents should not stop dual antiplatelet treatment without advice from their cardiologist
• Diabetes tablets and insulin may need adjusting while you eat less during bowel preparation; follow your written instructions
• Mention iron tablets, as your instructions may ask you to stop them for a few days before the test
• Bring a list of all your medicines, including vitamins and supplements, on the day

What do the different polyp types mean, and when is my next colonoscopy?

Most polyps are harmless, but some can slowly turn into cancer. Adenomas and sessile serrated lesions are the main types that can become cancerous. Small hyperplastic polyps in the lower bowel carry little risk. Your next colonoscopy depends on the type, size and number of polyps and on how complete the examination was.

Polyps are sent to a pathologist, who looks at them under a microscope. The report describes the type, whether there is any dysplasia (abnormal cell growth) and whether the polyp was removed completely. The intervals below come from the 2020 US Multi-Society Task Force recommendations and assume the whole bowel was seen with good preparation. Your own plan may differ with family history or other risks.

• Normal examination: repeat screening in about 10 years
• 1 to 2 tubular adenomas smaller than 10 mm: repeat in 7 to 10 years
• 3 to 4 adenomas smaller than 10 mm: repeat in 3 to 5 years
• 5 to 10 small adenomas, or any adenoma 10 mm or larger, or with villous features or high-grade dysplasia: repeat in 3 years
• More than 10 adenomas: repeat in 1 year, and genetic assessment may be considered
• Small hyperplastic polyps in the rectum or sigmoid colon: repeat screening in about 10 years
• 1 to 2 sessile serrated lesions smaller than 10 mm: repeat in 5 to 10 years; 10 mm or larger, or with dysplasia: repeat in 3 years
• An adenoma or sessile serrated polyp of 20 mm or more removed in pieces: repeat colonoscopy in about 6 months

How is the quality of a colonoscopy judged?

Specialist societies judge a colonoscopy by a few measures: whether the bowel was clean enough, whether the camera reached the start of the large bowel (the caecum), how carefully the lining was inspected on the way out, and how often precancerous polyps are found.

These measures are set by the American Society for Gastrointestinal Endoscopy and the American College of Gastroenterology. They apply across many examinations rather than to any one patient, but they explain why preparation matters so much and why the doctor takes time on the way out. You are entitled to ask whether your bowel preparation was adequate and whether the whole bowel was seen.

• Bowel preparation: adequate in at least 90 percent of examinations, because poor preparation hides polyps and may mean repeating the test sooner
• Caecal intubation: the whole large bowel reached and photographed in at least 95 percent of examinations
• Adenoma detection rate: at least 35 percent of people aged 45 and over should have one or more adenomas found; it is the best validated quality measure
• Withdrawal time: an average of at least 8 minutes spent inspecting the lining on the way out in normal examinations
• Complete polyp removal, with small polyps of 4 to 9 mm usually removed with a cold snare

What can be treated during a colonoscopy?

Polyps can be removed during the same procedure, tissue can be taken for testing, and some bleeding points can be controlled. If something is found that needs more than this, such as a cancer, Dr Wong explains it and sets out the next tests or surgery.

• Polypectomy to remove growths during the test
• Biopsy to diagnose inflammation or other conditions
• Bleeding control if needed
• A plan for follow-up or further treatment based on findings

What are the risks and complications?

Bloating and wind are common after colonoscopy. Less often there is minor bleeding after a biopsy or polyp removal, or a reaction to sedation. Perforation of the bowel is rare, and a very small lesion can occasionally be missed.

• Bloating and wind for a short time afterwards
• Minor bleeding after a biopsy or polypectomy
• Rarely, a tear or perforation of the bowel
• Reactions to sedation
• Missing a very small lesion, although the test is highly accurate