Colonoscopy: what it is, how to prepare and what happens on the day

The large bowel (colon) and rectum are the last part of the digestive tract. A colonoscope is a flexible tube, with a light and a small camera at its tip, that is passed through the back passage and guided along the bowel while the images appear on a screen. Cancer Research UK explains that the test looks at the whole of the inside of the large bowel, and that the person carrying it out is called an endoscopist, who may be a doctor or a specialist nurse.

Because the camera is inside the bowel, the endoscopist can do more than look. Small tissue samples (biopsies) can be taken for the laboratory, and growths called polyps can usually be removed on the spot with a wire loop passed through the tube. That is one reason the test is valued: it can both diagnose and treat.

NHS inform says people are invited for a colonoscopy after routine bowel screening results that need further investigation, or after a GP referral to investigate bowel symptoms. Cancer Research UK adds several other situations, listed below. Being offered a colonoscopy does not mean cancer has been found; most of the time the test does not find anything to worry about, as the NHS points out.

• To help find the cause of bowel symptoms.

• To look for early signs of bowel cancer as part of the national bowel cancer screening programme.

• To look for early signs of bowel cancer in people at high risk of developing it.

• As follow-up after treatment for bowel cancer.

• To look further into an abnormality found on another test, such as a CT scan.

You will be sent written instructions by the endoscopy unit, and Cancer Research UK stresses that it is important to read and follow them. NHS inform describes a typical pattern: a low fibre diet, usually from about 3 days before, then only clear liquids on the day before, plus a laxative powder mixed with water. Exact timings depend on the hospital and on which laxative you are given, so your own letter always overrides general guidance.

The laxative acts after a few hours and you will need the toilet often and suddenly, sometimes with tummy cramps. Cancer Research UK suggests staying at home for a few hours after taking it so that you are near a toilet, and NHS inform recommends planning where you will take it and rearranging work or caring responsibilities if you can.

Tell the unit about your medicines when you receive your appointment. Cancer Research UK says iron tablets or other medicines that can cause constipation might need to be stopped a week before, and that you should contact the endoscopy unit as soon as possible if you take blood-thinning medicines or have diabetes, because you will be given other instructions to follow.

You will usually have the test as an outpatient in the endoscopy department. A member of staff explains what will happen and asks you to sign a consent form, and you change into a hospital gown. You lie on your left side with your knees drawn up. Cancer Research UK says you are offered painkillers and may have medicine to relax you that can also make you drowsy, given through a small tube in a vein, while some hospitals use gas and air instead.

The colonoscope is passed into the bowel and a small amount of gas or water is introduced to open the bowel so the lining can be seen. NHS inform says this may make you feel slightly bloated and want to go to the toilet, although your bowel will be empty. Many people pass wind, which is normal. The endoscopist may press on your tummy or ask you to change position to help the tube pass around the bends.

Cancer Research UK says a colonoscopy can be uncomfortable but should not be painful. NHS inform mentions mild pressure, cramping and bloating as sensations people can expect, and says you can discuss the options for comfort with the team before the procedure begins.

After the test you go to a recovery area and are offered a snack and a drink. Cancer Research UK says bloating and cramping afterwards are due to the gas used and usually settle after a few hours. If you had sedation, NHS inform says you need someone to take you home and stay with you until the following day, and you should not drive for 24 hours. Cancer Research UK also lists not drinking alcohol, operating heavy machinery or signing legally binding documents during that time. If you chose gas and air instead, NHS inform says you can drive and go home without an escort.

Cancer Research UK describes colonoscopy as a very safe procedure and lists the possible risks: tummy pain, bleeding after a biopsy or polyp removal, a rare tear in the bowel wall (perforation) that would be likely to need surgery, and rare reactions to the sedation or pain relief, including an allergic reaction. The bleeding is usually a small amount that stops on its own within a couple of days. It advises contacting the endoscopy staff or the hospital 24 hour advice line if bleeding continues or is heavy. Use the contact details you are given, and call 999 or go to A&E if you feel seriously unwell.

Before you go home, the doctor or nurse should tell you whether any polyps were removed or biopsies taken. Cancer Research UK says biopsy results can take up to 2 weeks, while the NHS gives 2 to 3 weeks for the letter or call with your overall result and says a copy goes to your GP if they referred you.

The NHS describes three broad outcomes. A normal result means no polyps or cancer were found, though people at higher risk of bowel problems might be asked to have regular colonoscopies every few years. If polyps were found and removed, the report will say whether further treatment is needed or whether a check-up colonoscopy is advised in future. If cancer is found, the NHS says you will see a cancer specialist as soon as possible, and that the earlier anything is found, the easier and quicker it is to treat.

A colonoscopy can also confirm bowel conditions that are not cancer. The NHS lists piles, Crohn's disease, diverticular disease or diverticulitis and ulcerative colitis, and says to speak to a GP about next steps if you have one of these.

On the NHS, a colonoscopy usually follows a GP referral for symptoms or a positive screening test, and is carried out in a hospital endoscopy unit. Some people instead arrange a colonoscopy privately, usually for speed or choice of date and site. The test itself, the preparation and the safety information are the same in both settings, and in either case the result should be sent to your GP so your records stay complete. Ask whichever unit you attend how results will be communicated and who to contact if you have concerns afterwards.