Colon cancer symptoms and screening (bowel cancer)

Colon cancer starts in the large intestine (colon), which is the final part of your gut. 

What is colon cancer?

Colon cancer usually starts as small, noncancerous (benign) lumps of cells called polyps that form along the inside of the colon. Over time, some polyps can become cancerous, causing colon cancer, which is also known as bowel cancer.

Colon cancer can occur at any age but is most common in adults aged over 60. It is one of the most common types of cancer in the UK.

Colon cancer symptoms

Many of the symptoms of colon cancer overlap with other conditions. However, it is still important to see your doctor if you experience any of the symptoms of colon cancer, so the cause can be investigated and treated. If the cause is colon cancer, early detection significantly improves the chances of successful treatment. 

Colon cancer symptoms include abdominal pain or cramping, as well as: 

  • Any change in your bowel habits that persists for more than a few days eg constipation, diarrhoea and thinner stools
  • Bleeding from your back passage (rectum) — the blood will be bright red
  • Blood in your stools — your stools may consequently look darker brown or black
  • Fatigue and feeling weak
  • Feeling that you need to open your bowels even after you have recently passed stools
  • Unexplained weight loss

When to see a doctor

If you notice any of the above symptoms, you should see your doctor so that they can investigate your symptoms and if needed, provide appropriate treatment.

In the UK, the national bowel cancer screening programme is available to anyone aged 60 or over. However, if you have a family history of colon cancer or other risk factors, your doctor may recommend screening at a younger age.

Colon cancer screening

Colon cancer screening involves using a home test kit to collect a sample of your stools. This will be sent to a lab for analysis, where they will check for blood in your stools. If blood is detected in your stools, this can be a sign of polyps, which can later become cancerous, or colon cancer.

Colon cancer screening is important to catch colon cancer as early as possible, which improves the likelihood that treatment will be successful. Screening can also detect polyps, which can be removed before they become cancerous. Colon cancer is the fourth most common cancer in the UK.

Your doctor will advise you about what age you should start colon cancer screening, depending on your specific risk factors.

What is the procedure for bowel cancer screening?

The procedure for bowel cancer screening starts with collecting a stool sample for analysis in a lab. Depending on the results, you may not need any further tests. However, if blood is detected in your stools, you may need to have a colonoscopy

If you need to have a colonoscopy as part of your bowel cancer screening, you will be asked to fast the day before your procedure ie not eat any food. You will also be asked to take a strong laxative, usually given as a liquid that you drink or in some cases, as an enema that you insert into your rectum, before your procedure. This will empty your colon so that your doctor can more easily examine the inside of your colon. 

Types of colon cancer screening


A colonoscopy is an examination of the lining of your colon using a thin, flexible, tube-like telescope with a light and camera on the end (colonoscope), which is carefully passed through your rectum and into your colon. The procedure usually takes up to 30 minutes.

If no abnormalities are found and you are not at an increased risk of colon cancer, you will be invited back for bowel cancer screening in two years.

Stool DNA test

A stool DNA test involves collecting a sample of your stools using an at-home test kit for analysis in a lab. The lab will check for blood in your stools as well as for certain changes in the DNA of cells in your stools. Specific DNA changes can indicate the presence of colon cancer or an increased likelihood of certain cells becoming cancerous (precancerous conditions).

Stool DNA tests are usually repeated every two years.

Faecal occult blood test

The faecal occult blood test (FOBT) and faecal immunochemical test (FIT) check your stools for microscopic amounts of blood ie blood that is invisible (occult) to the eye.

These tests are usually repeated every two years.

Virtual colonoscopy (CT colonoscopy)

A virtual colonoscopy, also known as a CT colonoscopy, is less invasive than a traditional colonoscopy. It uses a CT scan to create images of the inside of your body, specifically focused on your colon and rectum. To ensure clear images are captured, a small tube (catheter) will be passed into your rectum, through which air or carbon dioxide will be passed. The procedure usually takes around 10 minutes.

Colon cancer symptoms and screening FAQs

At what age should I get a colonoscopy?

If you have symptoms of bowel cancer, such as bleeding from your rectum, a persistent change in your bowel habits, unexplained weight loss and/or feeling that you need to open your bowels even after recently doing so, then your doctor may recommend a colonoscopy. They may also recommend a colonoscopy if you have other bowel symptoms that need further investigation. If you do not have any symptoms, you will be invited for bowel cancer screening every two years from age 60, which may involve a colonoscopy. 

Can you check for colon cancer with a blood test?

A blood test can help diagnose colon cancer. However, for a definitive diagnosis, you will also likely need to have a colonoscopy, so that your doctor can see the inside of your colon. 

How accurate is a stool test for colon cancer?

The accuracy of stool tests for detecting colon cancer varies depending on the type of test. DNA stool tests are more accurate than stool tests that detect the presence of blood for picking up colon cancer. However, no screening test is 100% accurate, which is why if you notice any symptoms of bowel cancer, you should still see your doctor even if you recently had a negative stool test.  

We hope you've found this article useful, however, it cannot be a substitute for a consultation with a specialist

If you're concerned about symptoms you're experiencing or require further information on the subject, talk to a GP or see an expert consultant at your local Spire hospital.

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