CT colonography vs colonoscopy

Both tests aim to find polyps and cancers in the colon. Colonoscopy looks at the lining directly with a camera. CT colonography builds a three-dimensional model from CT slices, and radiologists "fly through" the bowel on screen.

Both need the bowel to be clean and distended. In CT colonography, tagging agents may be taken beforehand to mark residual stool so it is not mistaken for a polyp.

It is less invasive in feel: a small tube allows gas to inflate the bowel, and there is no sedation, so you can drive afterwards. It reaches the entire colon even in people whose bowel is tortuous or narrowed, and it is useful after a failed or incomplete colonoscopy. It also shows structures outside the bowel.

It tends to identify larger polyps and cancers well. Its sensitivity for small polyps is lower than colonoscopy, though small polyps are of limited clinical significance in many cases.

It cannot take biopsies or remove polyps, so a positive result generally leads to a second procedure. Flat lesions can be missed. Radiation is a consideration, particularly if repeated surveillance is planned. Incidental findings outside the colon, such as kidney or lung findings, may trigger more tests.

It also requires bowel preparation, and some people find the gas inflation uncomfortable. Dietary and laxative preparation continues to apply.

Guidelines treat colonoscopy as the first choice where a bowel examination is needed, because it diagnoses and treats at once. CT colonography is recommended where colonoscopy is incomplete, unsafe or not tolerated, a position reflected in UK imaging and endoscopy guidance and in NICE suspected-cancer pathways.

The decision also reflects capacity: CT colonography has been used to provide timely bowel imaging when endoscopy capacity is limited, with colonoscopy reserved for those with positive findings.

• Suitable for colonoscopy and no specific barriers: colonoscopy is usually preferred.

• Previous incomplete colonoscopy: CT colonography is commonly the next step.

• Frailty, heart or lung disease making sedation risky, or anticoagulation that cannot be stopped: CT colonography is often favoured.

• Concern about radiation, for example repeated surveillance in a younger person: colonoscopy is generally preferred.

• A polyp was seen on CT: expect colonoscopy for removal.

"Virtual" does not mean no preparation or no discomfort; bowel cleansing and gas inflation are still required. It is also not a screening replacement in the NHS programme, where FIT leads to colonoscopy.

And a normal CT colonography does not mean no polyps exist; very small or flat ones can be missed.