Diagnosing colon cancer

If you have symptoms of colon cancer, see your GP. They will examine you and if they think you may have colon cancer, will refer you to a specialist doctor. Some people may be diagnosed after doing a screening test for bowel cancer. 

The specialist doctor will examine you and ask about your general health, medical history and if any family members have had bowel cancer. They will also arrange for you to have tests to check your bowel.

The main test used to diagnose bowel cancer is a colonoscopy. A doctor or nurse uses a long flexible tube with a camera on the end to examine the inside of the large bowel. Other tests that may be done include:

  • a sigmoidoscopy, which looks inside the rectum
  • a CT scan, which uses x-rays to show a three-dimensional (3D) image of the bowel.

You should get the results of your tests within two weeks. Waiting for them can be a stressful time. Talking about your worries with someone close to you can help.

Getting diagnosed

If you have symptoms, you will usually begin by seeing your GP, who will examine you. If they think that your symptoms could be caused by cancer, they will refer you to a specialist doctor.

You should be seen at the hospital within 14 days. Some people are diagnosed after taking a test as part of the NHS Bowel Screening Programme. Bowel screening is a way of finding bowel cancer at an early stage before it causes symptoms.

In England, Wales and Northern Ireland, men and women aged 60 to 74 are invited to do a faecal occult blood test (FOB test) every two years. In Scotland, the ages range from 50 to 74. They are sent a home test kit, which is used to collect a poo (stool) sample. If you have a family history of bowel cancer, you may need to have bowel screening at an earlier age.

Sometimes people are diagnosed with colon cancer after going to hospital with a problem, such as bowel obstruction. This is when part of the bowel becomes blocked. It may cause symptoms such as tummy pain, nausea and vomiting, and constipation.

At the hospital

The doctor will ask you about your general health and any previous medical problems you have had. They will also ask whether you have a family history of bowel cancer.

They will examine you, and may do a rectal examination. This is when the doctor places a gloved finger into your back passage to feel for any lumps or swelling. It may feel uncomfortable, but is not usually painful.

You will usually have a blood test to check your level of red blood cells. If you have a low number of red blood cells, this is called anaemia. You will also have blood tests to check whether your liver and kidneys are working normally.

The main test used to look for bowel cancer is a colonoscopy.

Other tests that are sometimes used to diagnose bowel cancer include:

  • virtual colonoscopy
  • sigmoidoscopy.


A colonoscopy looks at the inside of the whole length of the large bowel. You can usually have this test as an outpatient. It takes about an hour.

Your bowel has to be completely empty for a colonoscopy. You will be told what you can eat and drink the day before the test. You will also take a medicine (laxative) to empty your bowel.

Just before the test, you may have a sedative as an injection into a vein (intravenously). This will help you feel more relaxed while you have the colonoscopy.

Once you are lying comfortably on your side, the doctor or nurse will gently pass a flexible tube into your back passage. This tube is called a colonoscope. There is a tiny light and camera on the end of it. During the test, the doctor or nurse will use this to photograph any areas of the bowel that look abnormal. They may also take samples (biopsies) from these areas. The biopsies will be sent to the laboratory to see if there are any cancer cells.

Most people are ready to go home a few hours after having a colonoscopy. You will need someone to collect you from the hospital, as you should not drive for 24 hours after a sedative.

Virtual colonoscopy (CT colonography, CT enema, CT pneumocolon)

In a virtual colonoscopy, a computerised tomography (CT) scanner takes a series of x-rays, which builds up a three-dimensional picture of your bowel. It is done in the hospital CT department and you can usually have it as an outpatient.

This test may be done instead of a colonoscopy, or it may be done if the colonoscopy didn’t give a clear enough picture.

Your bowel has to be completely empty for a virtual colonoscopy. You will need to follow a special diet for a few days and take a laxative before the test. Your hospital will tell you what to do.

Your doctor may give you an injection to help the muscles in your bowel relax. You may also have an injection of a dye (called a contrast medium) at the same time. Your doctor will tell you if you are going to have this.

Just before the scan, your doctor passes a tube into your back passage (rectum) and pumps in some air and gas (carbon dioxide).

This expands the bowel and helps to give a clearer picture. You will have two CT scans – one lying on your back and one lying on your front.


This test looks at the inside of the rectum and the part of the colon closest to the rectum (the sigmoid colon). You can usually have it as an outpatient.

A sigmoidoscope is a tube with a light and camera on the end. You lie curled on your left side and a doctor or nurse passes the tube into your back passage. A small amount of air is pumped into the bowel to make it easier to see inside it. This will make you feel like you need the toilet, but the feeling will gradually go away once the test is over.

During the test, the doctor or nurse will take samples of tissue (biopsies) from any areas of the colon that look abnormal. This is painless.

You should be able to go home as soon as the test is over.

Waiting for test results

Waiting for test results can be a difficult time. It may take from a few days to a couple of weeks for the results of your tests to be ready. You may find it helpful to talk with your partner, family or a close friend. Your specialist nurse or one of the organisations listed on our database, can also provide support. You can also talk things over with one of our cancer support specialists on 0808 808 00 00.

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