There are many diagnostic tests that your doctor may have to run through before delivering a diagnosis of colorectal cancer.
Medical history and physical exam
Your doctor will ask about your medical history to learn about possible risk factors, including your family history. You will also be asked if you’re having any symptoms and, if so, when they started and how long you’ve had them.
As part of a physical exam, your doctor will feel your abdomen for masses or enlarged organs, and also examine the rest of your body. You may also have a digital rectal exam (DRE). During this test, the doctor inserts a lubricated, gloved finger into your rectum to feel for any abnormal areas.
Tests to look for blood in your stool
If you are seeing the doctor because of symptoms you are having (other than bleeding from your rectum or blood in your stools), he or she may recommend a test to check your stool for blood that isn’t visible to the naked eye (occult blood), which might be a sign of cancer. These types of tests – a fecal occult blood test (FOBT) or fecal immunochemical test (FIT) – are done at home, and require you to collect 1 to 3 samples of stool from a bowel movement.
If you’ve already had an abnormal screening test, this should not be the next step. You should have a diagnostic colonoscopy (see below).
Blood tests
Your doctor might also order certain blood tests to help determine if you have colorectal cancer. These tests also can be used to help monitor your disease if you’ve been diagnosed with cancer. They will measure the different types of cells in your blood, as well as look for tumour biomarkers.
Colorectal cancer can lead to anemia (low red blood cell count) if the tumour has been bleeding for a long time. Biomarkers alone are not reliable enough to lead to a colorectal cancer diagnosis, as other medical conditions can lead to abnormal tumour marker levels.
Diagnostic colonoscopy
A diagnostic colonoscopy is just like a screening colonoscopy, but it’s done because a person is having symptoms, or because something abnormal was found on another type of screening test.
For this test, the doctor looks at the entire length of the colon and rectum with a colonoscope, a thin, flexible, lighted tube with a small video camera on the end. It is inserted through the anus and into the rectum and the colon. Special instruments can be passed through the colonoscope to biopsy or remove any suspicious-looking areas such as polyps, if needed.
A colonoscopy is a minor procedure, and can be done in a clinic or a doctor’s office.
Proctoscopy
This test may be done if rectal cancer is suspected. For this test, the doctor looks inside the rectum with a proctoscope, a thin, rigid, lighted tube with a small video camera on the end. It’s put in through the anus. The doctor can look closely at the inside lining of the rectum through the scope. The tumor can be seen, measured, and its exact location can be determined.
Biopsy
Usually if a suspected colorectal cancer is found by any screening or diagnostic test, it is biopsied during a colonoscopy. In a biopsy, the doctor removes a small piece of tissue with a special instrument passed through the scope. Less often, part of the colon may need to be surgically removed to make the diagnosis.
The only way to confirm the presence of colorectal cancer is to look at biopsy samples under a microscope.
Imaging tests to look for colorectal cancer
Imaging tests use sound waves, x-rays, magnetic fields, or radioactive substances to create pictures of the inside of your body. These tests can include CT scan (3D X-ray), ultrasound, MRI, and PET scan.
Information taken from American Cancer Society.