Diagnosis

Doctors use many tests to diagnose cancer and to determine if it has metastasized (spread). Some tests may also determine which treatments may be the most effective. For most types of cancer, a biopsy is the only way to make a definitive diagnosis of cancer. If a biopsy is not possible, the doctor may suggest other tests that will help make a diagnosis. Imaging tests may be used to find out whether the cancer has metastasized. Your doctor may consider these factors when choosing a diagnostic test:

• Age and medical condition
  
  
• The type of cancer suspected
  
  
• Severity of symptoms
  
  
• Previous test results

In addition to a physical examination, the following tests may be used to diagnose colorectal cancer. The doctor will also ask about the person's medical and family history and will likely order a full-bowel examination, such as a colonoscopy, described in the Screening section. If colorectal cancer is present, a complete diagnosis that accurately describes the location and spread of the cancer may not be possible until the tumor is surgically removed.

Biopsy. A biopsy is the removal of a small amount of tissue for examination under a microscope. Other tests can suggest that cancer is present, but only a biopsy can make a definite diagnosis of colorectal cancer. The sample removed from the biopsy is analyzed by a pathologist (a doctor who specializes in interpreting laboratory tests and evaluating cells, tissues, and organs to diagnose disease). A biopsy may be performed during a colonoscopy, or it may be done on any tissue that is removed during surgery. Sometimes, a CT scan or ultrasound is used to perform a needle biopsy (removing tissue through the skin with a needle that is guided into the tumor).

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 protein called carcinoembryonic antigen (CEA). High levels of CEA may indicate that a cancer has spread to other parts of the body. CEA is not an absolute test for colorectal cancer because it is elevated in only about 60% of people with colorectal cancer that has spread to other organs from the colon. In addition, other conditions may cause a rise in CEA. CEA tests are most often used to monitor patients already being treated for colorectal cancer. Learn more about tumor markers for colorectal cancer.

Imaging tests

Imaging tests performed before treatment look for cancer that may have spread outside of the colon and rectum.

CT scan. A CT scan creates a three-dimensional picture of the inside of the body with an x-ray machine. A computer then combines these images into a detailed, cross-sectional view that shows any abnormalities or tumors. Sometimes, a contrast medium (a special dye) is injected into a patient’s vein to provide better detail. In a person with colon cancer, a CT scan can check for the spread of cancer in the lungs, liver, and other organs.

Ultrasound. Ultrasound is a procedure that uses sound waves to produce images of the body to tell if cancer has spread to the liver or other organs. Endorectal ultrasound is commonly used to determine the depth of penetration of rectal cancer, and can be used to help plan treatment; however, this test cannot accurately detect metastatic lymph nodes (cancer that has spread to nearby lymph nodes) or metastatic disease beyond the pelvis.

Chest x-ray. An x-ray is a picture of the inside of the body. For instance, a chest x-ray can help doctors determine if the cancer has spread to the lungs.

Positron emission tomography (PET) scan. A PET scan is a way to create pictures of organs and tissues inside the body. A small amount of a radioactive substance is injected into a patient’s body and absorbed by the organs or tissues being studied. This substance gives off energy that is detected by a scanner, which produces the images.

Learn more about what to expect when having common tests, procedures, and scans.

Find out more about common terms used during a diagnosis of cancer.