Medical Oncologist
CION Cancer Clinics
Colonoscopy for diagnosis: what to expect and what comes next | CION Cancer Clinics
A colonoscopy is the test that confirms or rules out colon cancer when you have symptoms or a positive stool test. A thin camera looks at the whole lining of the large bowel, and samples are taken from anything abnormal. This page explains why it is needed, what the day involves, what the doctor may find, and what each finding leads to next. At CION Cancer Clinics, our team goes through your reports with you and plans the next test or treatment step.
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Medically reviewed by Dr. Naresh GunduConsultant Medical Oncologist · MBBS, DNB (Internal Medicine), DM (Medical Oncology, AIIMS) · last reviewed October 2026, next review due October 2027
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The short answer
Why is colonoscopy the test that confirms colon cancer?
Colonoscopy is the only test that both sees the inside of the colon and takes tissue from it. That tissue, called a biopsy, is what confirms or rules out cancer. Scans and stool tests can raise a suspicion, but they cannot settle it.
What it can do that a scan cannot
A CT scan sees the bowel from outside and can miss a small or flat growth. A colonoscopy sees the lining directly, in colour, from the back passage all the way round to where the small bowel joins. It can also remove polyps, small growths on the lining, during the same test. A scan never can.
How CION fits in
CION's oncologists decide whether you need a colonoscopy and what the endoscopist should look for. CION arranges the colonoscopy at an accredited partner hospital, and the report and biopsy result come back to your oncologist, who plans the next step with you.
A positive stool test or a raised CEA blood marker is a reason for colonoscopy. Neither one is a diagnosis.From preparation to home
What happens before, during and after a colonoscopy?
The mechanics are covered in detail in our colonoscopy, biopsy and polyp removal guide. Here is the short version.
Clearing the bowel
The day before, you drink a strong laxative and switch to clear fluids. Most people find this the hardest part. A clean bowel is what lets the doctor see small growths, so our page on bowel preparation is worth reading early.
Sedation
Most people are given sedation through a vein, so they feel drowsy and relaxed. Tell the team about blood thinners, diabetes medicines and iron tablets well before the day, because some need pausing.
The test itself
You lie on your left side while a thin, flexible tube is passed through the back passage. Air or gas opens the bowel so the lining can be seen. Cramping or pressure is common and passes.
Going home
You rest until the sedation wears off, then go home with an adult. Do not drive, sign papers or go back to work for the rest of that day. Some wind and bloating afterwards is normal.
Not sure whether this applies to you?
Ask an oncologistFour possible findings
What might the doctor find, and what happens next?
The endoscopist often tells you what they saw before you leave. That is a first impression. The biopsy result is the answer.
A normal colon
If the whole colon was seen clearly and nothing was found, colon cancer becomes a much less likely explanation for your symptoms. Your doctor will look at other causes, such as piles, a fissure or an irritable bowel. New or worsening symptoms still deserve a fresh check.
One or more polyps
Most polyps are not cancer. Many are removed during the same test and sent to the laboratory. The result decides whether anything more is needed and when your next colonoscopy should be.
A growth that looks suspicious
Several biopsies are taken, and the spot is often marked with a small ink tattoo so a surgeon can find it later. The doctor may say it looks like cancer. Treatment is still planned only after the biopsy confirms it.
What usually follows
- The biopsy result from the laboratory
- A CT scan of the chest, abdomen and pelvis
- Blood tests, including a starting CEA
- Review by the tumour board
The camera could not reach all the way
Poor preparation, a narrowed bowel or a tight bend can stop the tube. The unseen part must still be checked, by repeating the test or with a CT colonography scan. Read more on incomplete colonoscopy.
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After the colonoscopy: when to go to the emergency department
A little blood in the first stool after a biopsy or polyp removal is common. Severe or worsening tummy pain, a hard swollen belly, fever, vomiting, or passing a lot of blood or clots is not. Go to the nearest emergency department the same day and say you have recently had a colonoscopy. This applies even if it starts several days later.
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Commonly believed
Is a colonoscopy dangerous, or too painful to bear?
"A colonoscopy can spread the cancer."
There is no good evidence that colonoscopy or a biopsy spreads colon cancer. Leaving a growth undiagnosed is the real danger, because nothing can be planned until the biopsy is done.
"It is too painful to go through."
With sedation, most people are drowsy and remember little. Cramping and pressure are common and pass quickly. Many people say the bowel preparation the day before was the harder part.
"My bleeding is from piles, so I can skip it."
Piles are common and do bleed. They can also sit alongside a growth higher up the bowel. If a doctor has advised colonoscopy, they have decided piles may not be the whole explanation.
"A CT scan can replace the colonoscopy."
A CT can miss small and flat growths and cannot take a biopsy. CT colonography is a useful backup when colonoscopy is incomplete or unsafe, but anything it finds still needs a colonoscopy.
Being straight with you
What this page cannot tell you
It cannot tell you what your colonoscopy found or what your biopsy means. Only the endoscopist who saw your bowel and the pathologist who examined the tissue can answer that, and your oncologist puts those answers together. If you are waiting for a result, ask when and how you will receive it.
Who this page does not apply to
If you are well, with no symptoms, and booked for screening, read about screening colonoscopy instead. If you have severe tummy pain, a swollen belly and cannot pass stool or wind, that may be a bowel blockage and needs the emergency department, not a planned colonoscopy. If the growth is in the rectum, the rectal cancer pages apply.
When colonoscopy may not be the right first test
Very frail people, those with a recent heart problem, or those with a severe flare of bowel inflammation may need a different test or a short delay. Your doctor weighs the risk against what the test will tell you. If you are offered a scan instead, it is fair to ask why.
Bring your stool test result, any earlier colonoscopy report and a full list of your medicines to the appointment.Questions we are asked
Common questions about diagnostic colonoscopy
Do I need a colonoscopy if my stool test is positive?
Yes. A positive FIT means blood was found in the stool. It does not say why. Many positive results come from polyps, piles or other causes that are not cancer, but only a colonoscopy can show which. A positive stool test is never a diagnosis.
How long does a colonoscopy take?
The test itself is usually well under an hour. Expect to spend most of a morning or an afternoon at the hospital, including checking in, sedation and recovery. The bowel preparation the day before takes longer than the test, so plan to stay near a toilet.
Will I be awake during it?
Most people have sedation through a vein and are drowsy and relaxed. Some remember parts of it, most remember very little. Deeper anaesthesia is sometimes used. Ask in advance which is offered, and whether more can be given if it becomes uncomfortable.
Can the doctor tell me on the day if it is cancer?
The endoscopist can often say whether a growth looks suspicious, and many will tell you before you leave. That is an impression, not a diagnosis. Only the biopsy confirms cancer, and treatment is planned only once that result is back.
How long does the biopsy result take?
It varies with the laboratory and the number of samples, and extra tests on the tissue can add time. Before you leave, ask when the result is expected and who will explain it. If you hear nothing by then, call and ask rather than waiting.
Should I stop my blood thinner before the test?
Never stop it on your own. Some blood thinners need pausing before biopsy or polyp removal, and some are too risky to stop. The doctor who prescribed it and the colonoscopy team decide together. Tell them about every medicine, including aspirin.
Is colonoscopy safe for older parents?
Age alone is not a reason to avoid it. What matters is general health, heart and lung function, and whether the result would change treatment. For some frail people a CT colonography is gentler. The doctor weighs this with the patient and the family.
What happens after cancer is confirmed?
A staging CT, blood tests and further tests on the biopsy follow. The tumour board then reviews everything and recommends a plan. Call the CION helpline if you want help understanding a colonoscopy or biopsy report you already have.
Your Specialists
Meet CION's colon cancer specialists. Bring your colonoscopy, scan or biopsy report.
Our surgical and medical oncologists plan colon cancer care together: which operation, whether chemotherapy is needed, and the follow-up that comes after.
Medical Oncologist
Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
Medical Oncologist
Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
Medical Oncologist
Dr. Owais Mohammed
MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
Medical Oncologist
Medical Oncologist
Surgical Oncologist
Dr. Muralidhar Muddusetty
MBBS (AIIMS), MS (Surgery) (AIIMS), DNB (Surgical Oncology), MRCS (Edinburgh)
Surgical Oncologist
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Sources
- NHS — Colonoscopy
- NIDDK (NIH) — Colonoscopy
- NHS — Tests and next steps for bowel cancer
- American Cancer Society — Testing for Colorectal Cancer
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
What the endoscopist can tell on the dayHow much a first impression can and cannot sayWhy a biopsy is still needed after colonoscopyWhy tissue matters even when the growth looks obviousWhat tests will I need?The scans and blood tests that come after colonoscopyMyth: a colonoscopy is unbearableIf fear of the test is what is holding you back
Talk to us
Been told you need a colonoscopy, and not sure what happens after?
Tell us what led to the test and we will explain the next steps and arrange the colonoscopy if you need one. One helpline serves every CION centre.