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If something of concern turns up as a result of screening or if you have symptoms, you will need more tests to find out if you really have cancer and, if so, to see how far it has spread. Some of these tests are the same ones that are used for screening people who do not have symptoms. (See “How is colorectal cancer found?”)

Medical history and physical exam

Your doctor will ask you questions about your health, talk to you about your family’s medical history, and do a complete physical exam.

Blood tests

Your doctor might do certain blood tests to look for signs of colorectal cancer. People with colorectal cancer often have low red blood cell counts (anemia) because of bleeding from the tumor. You might also have blood tests to check your liver function because colorectal cancer can spread to the liver.

There are other substances (called tumor markers) in the blood that can help tell how well treatment is working. But these tumor markers are not used to find cancer in people who have not had cancer and who seem to be healthy. They are most often used for follow-up of people who have already been treated for colorectal cancer.


In a colonoscopy a thin, flexible, lighted tube (a colonoscope) is put into the colon through the rectum. This allows the doctor to look at the inside of the rectum and part of the colon for cancer or polyps. If something abnormal is found, a piece of it can be removed to send for tests (this is called a biopsy). Polyps can also be removed completely during a colonoscopy. This test may be uncomfortable because air is put into the colon, but it should not be painful. During the test, drugs are given to make you sleepy so you will probably not be aware that the test is going on at all. Because of this you must have someone to help you get home afterward.

In order to have a colonoscopy, your colon must be empty and clean, so you must use special laxatives the day before (often called a prep). Many patients say that the prep is the worst part of the colonoscopy.

More information about having a colonoscopy is in Frequently Asked Questions About Colonoscopy and Sigmoidoscopy.

Biopsy and lab test of samples

The tissue removed in a biopsy is sent to the lab where it is looked at under a microscope to see if cancer is present. Even though other tests might suggest colorectal cancer, a biopsy is the only way to know for sure.

Other lab tests may also be done on biopsy samples to reveal more details about the cancer. Doctors may look for certain gene changes in the cancer cells that might affect how the cancer is best treated.

Imaging tests

The tests described below make pictures of the inside of your body. Imaging tests may be done for many reasons. They may be done to help find out whether an area might be cancer, to learn how far cancer may have spread, or to help learn whether treatment is working.

Computed tomography (CT or CAT) scan

A CT scan uses x-rays to take many pictures of the body that are then combined by a computer to give a detailed picture. A CT scan can often show whether the cancer has spread to the liver, lungs, or other organs. CT scans take longer than regular x-rays. The patient has to lie still on a table while the CT scan is being done. A contrast dye may be put into a vein or a special drink to help outline the area being looked at. The dye can cause some flushing (redness and warm feeling). Some people are allergic and get hives or, rarely, more serious reactions like trouble breathing and low blood pressure. Be sure to tell the doctor if you have any allergies or if you ever had a reaction to any contrast dye used for x-rays.

CT scans can also be used to guide a biopsy needle into a tumor. For this to be done, the patient stays on the CT table while a radiologist moves a biopsy needle through the skin and toward the mass. A tiny piece of tissue or a thin cylinder of tissue is then removed and looked at under a microscope.


Ultrasound uses sound waves to make a picture of the inside of the body. Most people know about ultrasound because it is often used to look at a baby during pregnancy. This is an easy test to have. For most kinds of ultrasounds, the patient simply lies on a table while a kind of wand is moved over the skin of the belly.

Two special types of ultrasound might be used for people with colon or rectal cancer. In one, the wand that gives off sound waves is placed into the rectum to look for cancer there and to see if it has spread to nearby organs or tissues. In the other test, used during surgery, the wand is placed against the surface of the liver to see if the cancer has spread there.

Magnetic resonance imaging (MRI) scans

Like CT scans, MRIs show a cross-section of the body. But MRI uses radio waves and strong magnets instead of radiation to take pictures. As with CT scans, a contrast dye might be used, but it isn’t needed for all MRIs. MRI scans are sometimes useful in looking at places in the liver where cancer might have spread. They can also help the doctor learn rectal cancers have spread into nearby tissues. MRIs take longer than CT scans and the patient has to lie inside a narrow tube for the test. This can feel confining and upset people with a fear of closed spaces. The machine also makes thumping and buzzing noises, but some places give you headphones with music to block this out.

Chest x-ray

This test may be done to see whether colorectal cancer has spread to the lungs.

Positron emission tomography (PET) scan

In this test, a type of radioactive sugar is put into your vein. The sugar moves through the body and is taken in by the cancer cells. Then you are put into the PET machine where a special camera can detect the radioactivity. Because the cancer cells absorb large amounts of the sugar they show up on the pictures as dark "hot spots." PET looks at your whole body, and is useful when the doctor thinks the cancer has spread, but doesn't know where. Special machines are able to do both a PET and CT scan at the same time (called a PET/CT scan). This allows the doctor to compare hot spots on the PET scan with the more detailed pictures from the CT scan.


Angiography is an x-ray done to look at blood vessels. For this test, a thin tube (called a catheter) is put into a blood vessel and moved until it reaches the area to be studied. (The skin is numbed before the tube is put in.) Then a dye is pushed through the catheter and x-ray pictures are taken. When the pictures are done, the catheter is taken out. Surgeons sometimes use this test to show blood vessels next to cancer that has spread to the liver. This can help surgeons decide whether a cancer can be removed and if so, it can help in planning the operation.

Last Medical Review: 10/15/2014
Last Revised: 02/26/2016