If you have a second or third unclear FOB test, or an abnormal result, you will be given an appointment to see a specialist screening nurse. The appointment will be at your local screening centre and should be within a week of the letter telling you the FOB test result.

The nurse will explain what your result means and answer any questions you may have. The nurse will discuss having a colonoscopy and give you a detailed explanation of the procedure, and what the benefits and risks are. They will also assess whether you are fit to have a colonoscopy.

A colonoscopy isn’t appropriate for everyone. If you are not able to have a colonoscopy, you may be offered a different investigation, such as a barium enema. A barium enema is a special x-ray of the large bowel.

A colonoscopy is a way of examining the lining of the bowel from the inside. It's the most thorough and effective way of diagnosing bowel cancer. The colonoscopy can also see if there are any polyps in your bowel. Polyps are growths of cells. They aren’t cancer, but can sometimes change into cancer over a number of years.

For a colonoscopy the bowel has to be completely empty. This means following a special diet for a few days before your test. You will also need to take laxatives (which will be provided by the screening centre). You will be given instructions about your diet, and about exactly when to take your laxatives, by the screening centre.

Just before the colonoscopy, you may be given a sedative to help you feel more relaxed. Once you are lying comfortably on your side, the nurse or doctor will gently pass a thin flexible tube (a colonoscope) into your back passage. The tube is made up of flexible fibres. It can easily pass around curves of the bowel so that the lining of it can be examined. A tiny light and camera on the end of the tube is used to show any abnormal areas.

During the colonoscopy, photographs and samples (biopsies) of the cells on the inside of the large bowel can be taken. Most polyps can be painlessly removed, using a wire loop that is passed down the colonoscope. A colonoscopy can be uncomfortable, but the sedative will help you feel more relaxed.

The colonoscopy should take between 30 and 45 minutes. Most people are ready to go home a couple of hours after their test. You will need to arrange for someone to collect you from the hospital, as you shouldn’t drive for several hours after a sedative. You should also have someone with you for about 12 hours afterwards.

For most people a colonoscopy is a straightforward procedure and they will have no side effects. Rarely however, complications can happen, including:

Breathing or heart problems

Some people react to the sedative that is used during a colonoscopy. This can cause temporary breathing or heart problems. It is rare to have a serious problem and you will be monitored during the colonoscopy.

Not being able to see the whole bowel

Sometimes it’s not possible to see the whole bowel during a colonoscopy. This can happen because the bowel isn’t completely empty or because the colonoscope can’t pass round a bend in the bowel to reach the end. If this happens you may be asked to have another colonoscopy, or a barium enema.

Heavy bleeding

About 1 in every 150 people who have a colonoscopy will have heavy bleeding afterwards. If tissue samples (biopsies) are taken or polyps removed there is a risk that the area may bleed. If you notice a lot of bleeding you should contact the screening centre as soon as possible for further advice.

A perforated bowel

Rarely the colonoscope can make a hole (perforation) in the wall of your bowel. This happens to about 1 in 1500 people who have a colonoscopy. If your bowel is perforated you may need to have an operation to mend the hole.

Risk of death

In extremely rare cases, a person may die as a result of having a colonoscopy. This is very rare and only happens in around 1 in every 10,000 colonoscopies. The benefits of having a colonoscopy far outweigh this risk.

It's important to note that there is currently very little UK research evidence relating to bowel screening and most of the research is from the United States. The figures quoted in this section are based on data which has been collated by the Cancer Research UK Primary Care Education Research Group, on behalf of the NHS Cancer Screening programme.

The specialist doing your colonoscopy will tell you if they have removed any tissue samples (biopsies) or polyps from your bowel. If samples have been taken, you will have to wait up to three weeks for the results. This can be an anxious time for you and it may help to talk things over with a close friend or relative or a support organisation.

There are three different results you could get:

• a normal result
• polyps
• a cancer.

Normal result

This means that no polyps or cancer have been found in your bowel. About half of people that have a colonoscopy will get a normal result. There is a small chance that the colonoscopy may miss a cancer, so a normal result doesn’t guarantee that you don’t have cancer, or won’t develop it in the future. If your colonoscopy was normal you will be offered bowel cancer screening again in two years, as long as you are aged under 70. Those above the screening age will be able to request an FOB test kit.

Polyps

If one or more polyps are found during your colonoscopy they can usually be removed. This is known as a polypectomy and it can help to prevent bowel cancer developing. Around four in ten (40%) of people who have a colonoscopy following an abnormal FOB test will be found to have polyps.

The removed polyps will be looked at in the laboratory and you will be told if you are in a low-risk, intermediate (medium) or high-risk group. If your polyps are low-risk you will be offered bowel cancer screening again in two years. If your polyps are intermediate or high-risk you will be asked to have another colonoscopy in one or three years time.

Cancer

Only very few people (about one in ten, or 10%) who have a colonoscopy after an abnormal FOB test will be diagnosed with bowel cancer. If a cancer is found, you will be referred to a cancer specialist for treatment.

If cancer is found at its earliest stage, there is a very good chance of curing it. About nine out of every ten (90%) early stage bowel cancers can be successfully treated. However, not all bowel cancers that are found by having a colonoscopy can be cured.

Although having a colonoscopy is the best way to diagnose bowel cancer, there is a very small chance that the specialist will not see a cancer during the procedure. This may happen in around one out of every 20 people (5%). This can happen because the bowel wasn’t completely empty, or the colonoscopist wasn’t able to pass the colonoscope through the whole length of the bowel, or rarely, because the specialist missed the cancer during the investigation.