Colorectal Cancer: Tests After Diagnosis

July 05, 2018

Colorectal Cancer: Tests After Diagnosis

After a diagnosis of colorectal cancer, you will likely have other tests. These tests help your healthcare providers learn more about your cancer. They can help show if the cancer has grown into nearby areas or spread to other parts of the body. The test results help your healthcare providers decide the best ways to treat the cancer. If you have any questions about these or other tests, be sure to talk with your healthcare team.

The tests you may have can include:

  • Lab tests of biopsy or surgery samples

  • Blood tests

  • CT scan of the abdomen

  • MRI

  • Positron emission tomography (PET) scan

  • Ultrasound

  • Chest X-ray

Lab tests of biopsy or surgery samples

Colorectal cancer is most often diagnosed by removing a sample of a polyp or tumor. This is called a biopsy. It is often done during a colonoscopy. In some cases, colorectal cancer is diagnosed when surgery is done to remove a tumor. In either case, the biopsy or surgery samples are sent to a lab. They are looked at under a microscope for cancer cells.

Once cancer has been diagnosed, special lab tests may be done on the biopsy or surgery samples to see if the cancer cells have certain gene changes. These tests can help show if certain types of cancer drugs are likely to work well.

Blood tests

Complete blood count

A complete blood count, also known as a CBC, involves drawing a sample of your blood and checking it for:

  • The number of red blood cells

  • The number of white blood cells

  • The number of platelets

Information from the CBC will help your healthcare provider assess the possible effects of the cancer. For instance, a low red blood cell count may be a sign of anemia that’s caused by the tumor bleeding. The CBC is also important before and during treatment. For example, it's important that your platelet count is high enough before you have surgery. Having a low platelet count before surgery puts you at greater risk for bleeding. If you are getting chemotherapy, your CBC will be checked regularly. That’s important because chemo can affect your bone marrow, where new blood cells are made.

Blood chemistry tests

Blood tests can be done to measure chemicals in your blood to see how certain organs, such as your liver and kidneys, are working. They may also show a possible spread of the cancer to the liver. 

CEA blood test

Your healthcare provider may ask for a test that measures a protein in your blood called CEA or carcinoembryonic antigen. This protein is released into the blood by some colorectal tumors. Your healthcare provider is most likely to ask for this if you have already been diagnosed with colorectal cancer. A number of things besides cancer can increase the CEA. So testing for it is not a good way to look for cancer if you never had cancer before. But CEA tests can often be a good way to see if treatment is working. These tests can also look for signs that the cancer is coming back after treatment.

Imaging tests

CT scan

Your healthcare provider may ask for a CT scan to look for tumors in your colon and rectum, as well as in other parts of your body, such as your lungs or liver. The scan takes multiple X-rays of your body from many angles. While you lie on a table that moves into the center of the scanner, a thin X-ray beam rotates around you. As the pictures are taken, a computer makes detailed images of your internal organs. The healthcare provider may give you an injection of a contrast dye. The dye will help any tumors show up clearly on the scans. A complete CT scan takes several minutes. Metal objects can get in the way of the X-rays. So you will be asked to remove any jewelry. You may be asked to wear a hospital gown during the test. You won’t feel the scan. But some people feel uncomfortable having to lie so still during the test.


An MRI uses magnets, radio waves, and a computer to take pictures of the inside of your body. While CT scans usually create a clear picture of your internal organs, MRIs have some advantages. An MRI doesn’t use radiation, and it may not require an injection of a dye. The dye used for MRIs is not advised if you have kidney disease. Your healthcare provider may ask for an MRI if you are allergic to the contrast dye that is used for CT scans. Your healthcare provider may also ask for an MRI to help check lesions that showed up but were unclear during a CT scan.

MRIs are not painful. But they can take a long time to complete. They may take up to an hour. During that time, you will need to lie still on a table that is moved into a long narrow tube. Some people say the test makes them feel claustrophobic. If you have had a fear of enclosed spaces, called claustrophobia, in the past, talk with your healthcare provider before agreeing to the test. He or she may give you medicine called a sedative to help you stay calm during the test. If you have a sedative, you will need to allow some time after the test for the sedative to wear off. Newer, more open MRI machines can sometimes be used instead. But the images may not be as sharp in some cases. The equipment also makes loud banging noises during the procedure. You can ask for earplugs if you think the noise will bother you.

An MRI uses powerful magnets. So you won’t be allowed to have anything metal in the room. Even eyeglasses and ballpoint pens can become dangerous projectiles when the magnets are turned on. If you have any kind of metal implant, such as a heart valve or a joint pin, you may not be able to have an MRI. This depends on the type of metal. The MRI equipment can also affect implants, such as a pacemaker. 

PET scan

A PET scan can give the healthcare provider a better idea of whether an abnormal area seen on a CT scan or other imaging test is a tumor or not. If you have already been diagnosed with cancer, your healthcare provider may use this test to see if the cancer has spread to lymph nodes or other parts of the body. A PET scan can also be helpful if your healthcare provider thinks the cancer may have spread, but doesn't know where. A PET scan can give helpful information about your whole body. The picture is not as detailed as a CT scan, but it can be used along with a CT scan to look for tumors. 

For this test, you are injected with a sugar that has a mildly radioactive substance. Cancer cells absorb more of this sugar than normal cells, and the radioactive material shows up during the image from the scan. To have the scan, you’ll need to lie still on a table that is pushed into the PET scanner. This machine takes pictures that show where the sugar is in your body. The entire process may take several hours. A PET scan is painless and noninvasive. But if you’re sensitive to the sugar, you may have nausea, headache, or vomiting.


Ultrasound uses sound waves to make a picture of the inside of your body. A small instrument called a transducer gives off sound waves. It picks up the echoes as they bounce off body organs. Ultrasound can be used to look for tumors in your liver, gallbladder, pancreas, or elsewhere in your abdomen, but it can't look for tumors of the colon. For the exam, you lie on a table while the transducer is moved along the skin over the part of your body being checked. Usually, the skin is first greased or lubricated with gel. The test is painless and fairly quick. 

Two other kinds of ultrasound can be useful for colorectal cancer:

  • An endorectal ultrasound can show how deep a tumor is in your rectum. This is important for finding out how advanced your cancer is. It can also find lymph nodes that might be cancer. The healthcare provider puts a thin probe into your rectum through an instrument called a proctoscope. The entire exam takes 5 to 10 minutes. It may be uncomfortable, but it isn’t painful. To prepare, you will need an enema the morning of the procedure.

  • For an intraoperative ultrasound, the probe is put against your liver during surgery. This test can help show if colorectal cancer has spread to your liver.

Chest X-ray

To see if the cancer has spread to your lungs, the healthcare provider may ask for a chest X-ray. An X-ray uses an energy beam to make a picture of areas inside your body. The test is simple and routine. It is very effective in showing lung nodules. It can also help show if you have lung or heart disease. This makes it a standard test before any major surgery. Some healthcare providers may also ask for a chest CT scan because it is better at seeing if cancer has spread to the lungs.

Working with your healthcare provider

Your healthcare provider will talk with you about which tests you'll have. Make sure to get ready for the tests as instructed. Ask questions and talk about any concerns you have.



July 05, 2018


Clinical presentation, diagnosis, and staging of colorectal cancer. UpToDate

Reviewed By:  

Alteri, Rick, MD,Gersten, Todd, MD