Diagnosing Colorectal Cancer

If you have screening test results that suggest colorectal cancer or you have symptoms, your doctor must find out whether they are due to cancer or some other cause. To do this, your doctor will recommend one or more of the exams and tests to make a diagnosis: 

• 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 about the symptoms you’re having, if any. Your doctor will examine your body, which includes feeling your abdomen for any abnormalities. In some cases, a digital rectal exam (DRE) may be recommended. 

• Colonoscopy. A colonoscopy allows the doctor to look inside the entire rectum and colon. It involves the use of a thin, flexible tube with a light and camera on one end. The tube is passed into the rectum and the colon. You will receive a sedative and pain medication to make you more comfortable. These can be used to screen for cancer and/or to look for any additional signs of cancer. Suspicious polyps are removed for testing during this procedure.

• Blood tests. Colorectal cancer often bleeds into the large intestine or rectum, which can cause people with the disease to become anemic (lower than normal level of red blood cells). A complete blood count (CBC) test may be used to measure the number of red cells in the blood to determine if bleeding is occurring. Your doctor may recommend other blood tests to check liver function or to detect the levels of carcinoembryonic antigen (CEA), a protein that can indicate the cancer has spread. 

• Biopsy. A biopsy is the removal of a small amount of tissue for examination under a microscope by a laboratory specialist called a pathologist. It may be performed during a colonoscopy or on tissue that was removed during surgery. Sometimes, imaging may be used to help perform a needle biopsy, which removes tissue through the skin with a needle that is guided into the tumor. A biopsy is the only test that can make a definite diagnosis of colorectal cancer. 

If the test results show that cancer is present, additional testing will be needed to determine the stage of colorectal cancer. If the cancer is recurring, these tests may be repeated to see if the cancer has spread to new areas of the body.

• Biomarker testing of the tumor. Also called molecular testing of the tumor, this involves running laboratory tests on a tumor sample to identify specific genes, proteins, and other factors unique to the tumor. The results of these tests can influence the recommended treatment options available to recurrent colorectal cancer patients or those who are diagnosed at a late stage.

• Computed tomography (CT) scan. A CT scan takes pictures of the inside of the body using X-rays taken from different angles. These pictures are then combined by a computer into a detailed, 3-dimensional image that shows any abnormalities or tumors. A special dye, called a contrast medium, may be given before the scan to provide better detail on the image. 

• Positron emission tomography (PET). During a PET scan, a small amount of a radioactive sugar substance is injected into a vein with the purpose of seeking out cancer cells. The PET scanner rotates around the body and makes a picture of where glucose is being used in the body. Malignant tumor cells show up brighter in the picture because they are more active and take up more glucose than normal cells do. PET scans are sometimes combined with a CT scan (PET-CT).  

• Magnetic resonance imaging (MRI). An MRI uses magnetic fields, not X-rays, to produce detailed images of the body. A special dye called a contrast medium is given — either orally or as an injection — before the scan to create a clearer picture. MRI is the best imaging test to find where the colorectal cancer has grown.

• Endorectal ultrasound. An ultrasound uses sound waves to create a picture of the internal organs to find out if cancer has spread. For colorectal cancer, endorectal ultrasound is the most common method used. A small, lubricated probe is interested in the rectum, and an image is produced on a screen. The image(s) help determine the size and where the cancer is in the rectum. 

• Chest x-ray. An x-ray may be done to see if the cancer has spread to the lungs.