Colorectal Cancer: Tests After Diagnosis

After a diagnosis of colorectal cancer, you'll likely need more tests. These tests help your healthcare providers learn more about the cancer. They can help show if it has grown into nearby tissues or spread to other parts of your 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, 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 belly (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 taking out and testing a small piece of a polyp or tumor. This is called a biopsy. The tissue removed is called a sample.

When cancer was found, more tests may have been done on the sample. For instance, certain lab tests are used to look for gene changes in the cancer cells. These tests can help learn details about the cancer and show if certain types of treatment should be used.

Blood tests

Complete blood count

A complete blood count (CBC) is done by taking a blood sample and checking it for:

  • The number of red blood cells

  • The number of white blood cells

  • The number of platelets

Information from the CBC can help your healthcare provider know what the cancer is doing to your body. For instance, a low red blood cell count (anemia) may be a sign that the tumor is causing bleeding.

The CBC is also important during treatment and will be checked often. For instance, 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're getting chemotherapy (chemo), your CBC needs to be watched. That's 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, like 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 might do a test that measures a protein in your blood called CEA (carcinoembryonic antigen). This protein is released into the blood by some colorectal tumors. You might hear this called a tumor marker test. Tracking CEA levels can often be a good way to see if treatment is working. The amount of CEA in your blood should drop as more and more cancer cells are killed. After treatment, CEA levels can still be tracked to see if they start to go up. This may be a sign that the cancer is coming back.

Imaging tests

CT scan

Your healthcare provider may ask for a CT scan to look for tumors in your colon and rectum. They may also want the scan to look for tumors in other parts of your body, such as your lymph nodes, lungs, or liver. The scan takes X-rays of your body from many angles. You lie on a table that moves into the center of the ring-shaped scanner, and a thin X-ray beam rotates around you. As the pictures are taken, a computer makes detailed images of your insides. You might be given an injection of a contrast dye through an IV (intravenous) line into your vein. The dye helps any tumors show up clearly on the scans. A complete CT scan takes a few minutes. You may have to wear a hospital gown during the test. You won’t feel the scan. But some people feel uncomfortable having to lie still on a narrow table during the test.


An MRI uses magnets, radio waves, and a computer to take pictures of the inside of your body. An MRI doesn’t use radiation, and dye may not be needed. Your healthcare provider may also ask for an MRI to help check lesions that showed up but were unclear during a CT scan. It might be used to look for cancer in the liver, brain, and spinal cord.

MRIs aren't painful. But they can take a long time to complete. They may take up to an hour. During that time, you'll need to lie flat and still on a table that's moved into a long narrow tube. If you don't like closed spaces, this test might be hard to do. Talk with your provider about this. The equipment makes loud banging noises during the scan. You can ask for earplugs if you think the noise will bother you.

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 cancer. This test can help 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 isn't 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 slowly goes through the big rings of a PET scanner. This machine takes pictures that show where the sugar is in your body. The whole process may take a few hours. A PET scan is painless. But if you’re sensitive to the sugar, you may have nausea, headache, or vomiting.


Ultrasound uses sound waves to make pictures of the inside of your body. A small tool called a transducer is moved over your skin. It gives off sound waves. The computer picks up the echoes as they bounce off body organs. Ultrasound can be used to look for tumors in your liver, gallbladder, pancreas, or other places in your belly. 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:

  • Endorectal ultrasound.  This can show how deep a tumor is in your rectum. It can also find lymph nodes that might be cancer. This is important for finding out how advanced your cancer is. A thin probe is put into your rectum through a tool called a proctoscope. The full exam takes 5 to 10 minutes. It may be uncomfortable, but it isn’t painful. To get ready, you'll need an enema the morning of the procedure.

  • Intraoperative ultrasound. For this, 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

A chest X-ray can be used to see if the cancer has spread to your lungs. An X-ray uses radiation to make a picture of the inside your body. The test is simple. It's very good at showing lung nodules. It can also help show if you have lung or heart disease.

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.

Online Medical Reviewer: Kimberly Stump-Sutliff RN MSN AOCNS
Online Medical Reviewer: Louise Cunningham RN BSN
Online Medical Reviewer: Todd Gersten MD
Date Last Reviewed: 2/1/2021
© 2022 The StayWell Company, LLC. All rights reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare provider's instructions.