Diagnosis of Colorectal Cancer

Early diagnosis of colorectal cancer gives you the best chance of curing your colorectal cancer. Here is some of the most important ways to diagnose colorectal cancer:

Blood tests

Because colorectal cancer often bleeds into the large intestine or rectum, people with the disease may become anemic. A test of the number of red cells in the blood, which is part of a complete blood count (CBC), can indicate that bleeding may be occurring.

Another blood test detects the levels of a CEA protein. High levels of CEA may indicate that a cancer has spread to other parts of the body.

Stool tests

Stool tests check for hidden, or occult, blood in the stool.

Double-contrast barium enema

This is an imaging test that uses a special substance (barium sulphate) and x-rays to make images of the entire large intestine (cecum, colon, rectum and anus). Air is pumped into the intestine to improve the view.

A double-contrast barium enema may be done when there is a positive fecal occult blood test or you have symptoms of colorectal cancer. It can show polyps or growths in the colon.


Sigmoidoscopy is used to look at the sigmoid colon (the last part of the colon) and rectum. A flexible sigmoidoscopy may be done if you have symptoms of colorectal cancer, such as changes in bowel habits, blood in the stool or abdominal pain.

Sigmoidoscopy may be done to measure the distance a rectal tumour is from the anus. Tumours that are lower in the rectum and close to the anus may need radiation therapy before surgery.


Colonoscopy is the main test used to diagnose colorectal cancer. During a colonoscopy, the doctor looks inside the colon and rectum using a flexible tube with a light and lens on the end (colonoscope).

A colonoscopy is done if you have blood in your stool or other symptoms of colorectal cancer. It is also used to check polyps or other abnormal areas.


During a biopsy, the doctor removes tissues or cells from the body so they can be tested in a lab. A biopsy is usually done during a colonoscopy or sigmoidoscopy to remove polyps or small amounts of tissue from the colon or rectum.

CT scan

A CT scan can be used to measure the tumor’s size. It can also check for the spread of cancer in the lungs, liver, and other organs. It is often done before surgery.


Transrectal ultrasound (TRUS) is an ultrasound technique in which the ultrasound transducer, or probe, is placed inside the rectum. It is used to find out the stage of the rectal cancer, including how deep the tumour has grown into the wall of the rectum. TRUS may also be used to measure how far a rectal tumour is from the anus.


MRI of the pelvis may be used to find out the stage of rectal cancer and where cancer has spread within the pelvis, such as to the lymph nodes. It may also be used to measure how far a rectal tumour is from the anus.

PET-CT scan

A positron emission tomography (PET) scan uses radioactive materials called radiopharmaceuticals to look for changes in the metabolic activity of body tissues. A PET-CT scan combines images from a PET scan and a CT scan may be used to help with staging colorectal cancer or plan surgery for metastatic tumours.




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