What are the symptoms of colorectal cancer?
The development of colorectal cancer from a polyp may take between 5–10 years, and early on there may be no symptoms at all. The most common symptoms are bleeding from the bowel, a change in bowel habit, such as unusual episodes of diarrhea or constipation, or an increase in the amount of mucus in the stool. A colorectal cancer can enlarge so that it partially or completely blocks the bowel leading to abdominal pain, constipation, and bloating. Sometimes tiny amounts of bleeding may go unnoticed, but may result in the development of anemia, which may cause tiredness and a decreased ability to work and exercise.

Some of these symptoms are similar to those of irritable bowel syndrome and this can sometimes cause difficulty in making a diagnosis. A prolonged change in bowel habit lasting more than two or three months should always be investigated, and rectal bleeding is not a symptom of irritable bowel syndrome.

What tests confirm a diagnosis of colorectal cancer?
Sometimes, the doctor will be able to detect a lump in the abdomen or on rectal examination but usually tests are needed. The most commonly used are:

Barium enema x-ray—after taking laxatives to empty the colon, it is filled with a combination of barium and air to outline its lining.

Flexible sigmoidoscopy—after an enema, a flexible telescope is passed through the anus, into the rectum and this can reach the lowest half of the colon.

Colonoscopy—like a barium enema, this requires laxatives to clear out the bowel. A flexible telescope is passed through the anus into the rectum but the tube is long enough to examine the entire large bowel. The procedure is a little uncomfortable and most patients are offered an injection to ease any discomfort.

CT scanning—this x-ray procedure is a relative newcomer and obviously has the advantage, (which many people appreciate) of not involving a tube being passed through the anus. It is not yet as reliable as colonoscopy but its quality is steadily improving and it seems likely to be used increasingly often.

Both flexible sigmoidoscopy and colonoscopy have the advantage that a small sample or biopsy can be taken to look at under the microscope. The above tests are used in slightly different situations depending upon the symptoms that patients may have and the availability of the investigations.