Colorectal Cancer

How is colorectal cancer diagnosed?

February 7, 2024 Word for Word Media 0Comment

Dr Daniel Surridge explains the process of how colorectal cancer is screened and diagnosed.


You can listen to this article below, or by using your favourite podcast player at pod.link/oncologybuddies

Since colorectal cancer has very few symptoms, it can be very hard to diagnose without specialised tests. You may not know that you have it until it’s too late. For this reason, it’s a good idea to screen for this cancer from the age of 45. Screening is performed in patients who don’t have symptoms but are at risk for this cancer.

Screening for colorectal cancer

There are several ways screening can be done. Your doctor may start with a stool test looking for microscopic traces of blood. You’ll need to supply a fresh stool sample to a laboratory where they can perform the test. If the test result is negative for blood, your chances for having a cancer are low. If it’s positive, the risk is high. However, other conditions can also cause bleeding in the colon. In order to diagnose the cause of bleeding, you’ll need to have a colonoscopy if you have a positive test.

Colonoscopy is a test where a camera is inserted into the anus and along the whole length of the colon to look for any tumours. The colon must be cleaned out before such a procedure so that the surgeon can see inside the colon. For this reason, a very strong laxative (bowel prep) is given before the colonoscopy.

If a growth is found and it’s early, it can be removed by the colonoscope and sent to the laboratory to make sure what it was and that all of it is out. But, if the colonoscopy doesn’t show any growths, then the stool test may have been falsely positive. If a cancer is found, a small piece will be taken and sent to the laboratory to confirm the diagnosis.

There are times when a colonoscopy isn’t possible, or it fails due to not being able to get the colonoscope to the end of the colon. When this happens, a CT colonography can be performed. This is a type of digital colonoscopy performed by a CT scan. You’ll still need to take the bowel prep beforehand. Unfortunately, a CT colonography can’t take a biopsy. So, if a growth is found, then a biopsy will still need to be performed surgically.

Important to note

A tumour marker test done on the blood can’t diagnose or rule out colorectal cancer. This test looks for a substance that is secreted by the tumour but it’s very unreliable for diagnosis. It can be used to look for recurrence of a cancer but not for screening or diagnosis.

Newer blood test-based techniques are being developed but are not readily available yet. These are called liquid biopsies and look for cancer DNA in the bloodstream.

Diagnosing colorectal cancer

Sometimes, your doctor may feel that the stool test isn’t necessary, and a colonoscopy might be needed up front. If anything is found during this procedure, then a biopsy will be taken, or the lesion removed to confirm the diagnosis.

If you do have symptoms, go and see your surgeon for a colonoscopy to make a diagnosis. There are occasions where colorectal cancer is seen incidentally on a scan or at an operation done for a different reason. This may prompt your surgeon to request more tests.

Once the diagnosis of cancer has been made, your surgeon will need to stage the cancer by performing various CT and MRI scans. These will allow your surgeon to know how big the cancer is and how far it has spread. This information is then discussed with a multi-disciplinary team of surgeons, oncologists, radiation oncologists and radiologists to work out the best plan possible to offer you.

Dr Daniel Surridge

MEET THE EXPERT – Dr Daniel Surridge


Dr Daniel Surridge is a subspecialist colorectal and robotic surgeon in Gauteng. He heads up the Joburg Colorectal Unit at Netcare Milpark Hospital. He was formerly the head of colorectal surgery at Chris Hani Baragwanath Academic Hospital which he ran for over seven years, and he has travelled extensively to acquire advanced skills and approaches to colorectal surgery.


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