Tests for Colorectal Cancer

Colorectal cancer is often found after symptoms appear, but most people with early colorectal cancer don’t have symptoms of the disease. This is why it’s important to get screened for colorectal cancer (see American Cancer Society Recommendations for Colorectal Cancer Early Detection) before any symptoms develop.

If your doctor finds something suspicious during a screening exam, or if you have possible symptoms of colorectal cancer, your doctor will recommend exams and tests to find the cause.

Medical history and 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 if you’re having any symptoms and, if so, when they started and how long you’ve had them.

As part of a physical exam, your doctor will feel your abdomen for masses or enlarged organs, and also examine the rest of your body. You may also have a digital rectal exam (DRE). During this test, the doctor inserts a lubricated, gloved finger into your rectum to feel for any abnormal areas. He or she may also test your stool to see if it contains blood that isn’t visible to the naked eye (occult blood).

Blood tests

Your doctor might also order certain blood tests to help determine if you have colorectal cancer. (These tests also can be used to help monitor your disease if you’ve been diagnosed with cancer.)

Complete blood count (CBC): This test measures the different types of cells in your blood. It can show if you have anemia (too few red blood cells). Some people with colorectal cancer become anemic because the tumor has been bleeding for a long time.

Liver enzymes: You may also have a blood test to check your liver function, because colorectal cancer can spread to the liver.

Tumor markers: Colorectal cancer cells sometimes make substances called tumor markers that can be found in the blood. The most common tumor markers for colorectal cancer are carcinoembryonic antigen (CEA) and CA 19-9.

Blood tests for these tumor markers can sometimes suggest someone might have colorectal cancer, but they can’t be used alone to screen for or diagnose cancer. This is because tumor marker levels can sometimes be normal in someone who has cancer and can be abnormal for reasons other than cancer.

Tumor markers are used most often along with other tests to monitor patients who already have been diagnosed with colorectal cancer. They may help show how well treatment is working or provide an early warning that a cancer has returned.

If symptoms or the results of the physical exam or blood tests suggest that you might have colorectal cancer, your doctor could recommend more tests. This most often is colonoscopy, but sometimes other tests may be done first.

Colonoscopy

For this test, the doctor looks at the entire length of the colon and rectum with a colonoscope, a thin, flexible, lighted tube with a small video camera on the end. It is inserted through the anus and into the rectum and the colon. Special instruments can be passed through the colonoscope to biopsy or remove any suspicious-looking areas such as polyps, if needed.

Colonoscopy may be done in a hospital outpatient department, in a clinic, or in a doctor’s office.

To learn more about colonoscopy, how it’s done, and what to expect if you have one, see Frequently Asked Questions About Colonoscopy and Sigmoidoscopy

Biopsy

Usually if a suspected colorectal cancer is found by any screening or diagnostic test, it is biopsied during a colonoscopy. In a biopsy, the doctor removes a small piece of tissue with a special instrument passed through the scope. Less often, part of the colon may need to be surgically removed to make the diagnosis. See Testing Biopsy and Cytology Specimens for Cancer to learn more about the types of biopsies, how the tissue is used in the lab to diagnose cancer, and what the results may show.

Lab tests of biopsy samples

Biopsy samples (from colonoscopy or surgery) are sent to the lab where they are looked at closely. Other tests may suggest that colorectal cancer is present, but the only way to be sure is to look at the biopsy samples under a microscope.

If cancer is found, other lab tests may also be done on the biopsy specimens to help better classify the cancer.

Gene tests: Doctors may look for specific gene changes in the cancer cells that might affect how the cancer is best treated especially if the cancer has spread (metastasized). For example, doctors now typically test the cells for changes in the KRAS and NRAS  and BRAF genes. Some doctors may also test for changes in the BRAF gene. Patients whose cancers have mutations in these genes typically do not benefit from treatment with certain targeted anti-cancer drugs.

MSI and MMR testing: Colorectal cancer cells are typically tested to see if they show high levels of gene changes called microsatellite instability (MSI). Testing might also be done to see if the cancer cells have changes in any of the mismatch repair (MMR) genes (MLH1, MSH2, MSH6, and PMS2). 

Changes in MSI or in MMR genes (or both) are often seen in people with Lynch syndrome (HNPCC). Most colorectal cancers do not have high levels of MSI or changes in MMR genes. But most colorectal cancers that are linked to Lynch syndrome do.

There are 2 possible reasons to test colorectal cancers for MSI or for MMR gene changes:

  • To identify patients who should be tested for Lynch syndrome. A diagnosis of Lynch syndrome can help plan other cancer screenings for the patient (for example, women with Lynch syndrome may need to be screened for uterine cancer). Also, if a patient has Lynch syndrome, their relatives could also have it, and may want to be tested for it.
  • To determine treatment options for colorectal cancer, where MSI or MMR results could change the way it is treated.

For more on lab tests that might be done on biopsy samples, see Understanding Your Pathology Report: Colon Pathology.

Imaging tests to look for colorectal cancer

Imaging tests use sound waves, x-rays, magnetic fields, or radioactive substances to create pictures of the inside of your body. Imaging tests may be done for a number of reasons, such as:

  • To look at suspicious areas that might be cancer,
  • To learn how far cancer has spread
  • To help determine if treatment is working

Computed tomography (CT or CAT) scan

A CT scan uses x-rays to make detailed cross-sectional images of your body. This test can help tell if colon cancer has spread into your liver or other organs.

Ultrasound

Ultrasound uses sound waves and their echoes to create images of the inside of the body. A small microphone-like instrument called a transducer gives off sound waves and picks up the echoes as they bounce off organs. The echoes are converted by a computer into an image on a screen.

Abdominal ultrasound: For this exam, a technician moves the transducer along the skin over your abdomen. This test 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.

Endorectal ultrasound: This test uses a special transducer that is inserted into the rectum. It is used to see how far through the rectal wall a cancer has grown and whether it has reached nearby organs or tissues such as lymph nodes.

Intraoperative ultrasound: This exam is done during surgery. The transducer is placed directly against the surface of the liver, making this test very useful for detecting the spread of colorectal cancer to the liver. This allows the surgeon to biopsy the tumor, if one is found, while the patient is asleep.

Magnetic resonance imaging (MRI) scan

Like CT scans, MRI scans show detailed images of soft tissues in the body. But MRI scans use radio waves and strong magnets instead of x-rays. A contrast material called gadolinium may be injected into a vein before the scan to see details better.

MRI can be used to look at abnormal areas in the liver or the brain and spinal cord that could be cancer spread. 

Endorectal MRI: MRI scans can be used in patients with rectal cancers to see if the tumor has spread into nearby structures. This can help plan surgery and other treatments. To improve the accuracy of the test, some doctors use endorectal MRI. For this test the doctor places a probe, called an endorectal coil, inside the rectum. This stays in place for 30 to 45 minutes during the test and can be uncomfortable.

Chest x-ray

An x-ray may be done after colorectal cancer has been diagnosed to see if cancer has spread to the lungs.

Positron emission tomography (PET) scan

PET scans  usually use a form of radioactive sugar that is put into the blood. Body cells take in different amounts of the sugar, depending on how fast they are growing. Cancer cells, which grow quickly, are more likely to take up larger amounts of the sugar than normal cells. A special camera is used to create a picture of areas of radioactivity in the body.

The picture from a PET scan is not as detailed as a CT or MRI scan, but it provides helpful information about whether abnormal areas seen on these other tests are likely to be cancer or not.

If you have already been diagnosed with cancer, your doctor 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 useful if your doctor thinks the cancer may have spread but doesn’t know where.

PET/CT scan: Some machines can do both a PET and CT scan at the same time. This lets the doctor compare areas of higher radioactivity on the PET scan with the more detailed picture of that area on the CT scan. 

Angiography

Angiography is an x-ray test for looking at blood vessels. A contrast dye is injected into an artery, and then x-rays are taken. The dye outlines the blood vessels on x-rays.

If your cancer has spread to the liver, this test can show the arteries that supply blood to those tumors. This can help surgeons decide if the liver tumors can be removed and if so, it can help plan the operation. Angiography can also help in planning other treatments for cancer spread to the liver, like embolization.

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master’s-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

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Last Medical Review: October 15, 2016 Last Revised: May 24, 2017

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