How to test for and diagnose colorectal cancer

This page was reviewed under our medical and editorial policy by

Maurie Markman, MD, President, Medicine & Science

This page was reviewed on September 8, 2022.

Estimated number of 2022 colorectal cancer cases in total and by colon and rectal cancer
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A thorough and accurate cancer diagnosis is the first step in developing a treatment plan for colorectal cancer (which also may be referred to as colon cancer or rectal cancer). Your multidisciplinary team of colorectal cancer experts use a variety of tests and tools designed for diagnosing colorectal cancer, evaluating the disease and planning your individualized treatment. Throughout your treatment, we'll use imaging and laboratory tests to track the size of the tumors, monitor your response to treatment and modify your plan when needed.

To accurately diagnose colorectal cancer, your care team may recommend one or several different types of tests. Each test can reveal different types of information, such as the location of the cancer cells, how fast they’re growing, or whether they’re affecting nearby tissues or organs. Just as your treatment plan is highly individualized, your diagnostic testing will be, too.

Your care team will recommend a test or set of tests based on:

  • Symptoms
  • General health
  • Age
  • Family history
  • Results of previous medical tests
  • Type of cancer suspected

Some of these tests may be used during both diagnosis and treatment to assess your progress.

Examples of procedures used to test for and diagnose colorectal cancer include:

Endoscopic procedures

Procedures such as a colonoscopy allow a doctor to examine the body from the inside. Doctors insert an endoscope or colonoscope—thin tubes with a camera at the end—into the body to look for polyps or other abnormalities. In some cases, colon polyps may be removed during a colonoscopy.

A colonoscopy may be used to help diagnose cancer in the colon or rectum.

An endoscopy that examines only the rectum or lowest part of the colon is called a proctoscopy. It’s similar to a colonoscopy but focuses only on the rectum instead of the entire colon and rectum. A proctoscope—another type of thin tube with a camera at the end—is inserted through the anus. From there, your doctor can identify cancerous cells, measure a tumor, and gain important information to formulate a treatment plan.

Learn more about colonoscopies and other endoscopic procedures used to diagnose colorectal cancer

Lab tests

There are generally two types of lab tests for colorectal cancer: stool tests and blood tests.

Stool tests

Stool tests that are conducted to look for blood, DNA abnormalities or other markers that may indicate cancer. They aren’t as invasive as a colonoscopy or proctoscopy. However, if stool test results suggest cancer may be present, you’ll likely require one of these procedures to confirm and diagnose cancer. 

A major benefit of stool tests is that they can be done at home. You would need to follow the instructions carefully and reach out to your care team with any questions. 

There are three main types of stool tests used to diagnose colorectal cancer:

Guaiac-based fecal occult blood test (gFOBT) requires multiple samples, and you may need to limit certain medications or foods before collecting the samples, including nonsteroidal anti-inflammatory drugs (NSAIDs, such as ibuprofen) and red meats.

Fecal immunochemical test (FIT) also requires multiple samples, but you don’t need to limit any medications or foods beforehand.

DNA stool test searches for specific sections of abnormal DNA that may indicate cancerous mutations, instead of looking for tiny amounts of blood in your stool. DNA stool tests don’t require limiting medications or foods beforehand. 

Blood tests 

Cancer cannot be diagnosed by blood tests alone, but they are one tool your care team can use to help make a diagnosis. 

Blood tests can help inform your care team on the following situations:

  • If you may be anemic due to a tumor causing bleeding. This internal bleeding may lead to a low red blood count. 

  • If cancer may have spread to your liver. A blood test can show a change in liver enzymes that may be due to cancer. 

  • If cancer cells are producing proteins called tumor markers. For colorectal cancer, the most common marker is called carcinoembryonic antigen (CEA).

Learn more about lab tests used to diagnose colorectal cancer


These tests may be critical in helping diagnose colorectal cancer. A gastroenterologist performs a biopsy by retrieving polyps and other tissue samples from the colon or rectum during a colonoscopy. Tissue samples may also be retrieved during other endoscopic procedures, such as a sigmoidoscopy or endoscopic ultrasound. The polyps and samples are then sent to a laboratory to be analyzed under a microscope to check for cancer cells.

Imaging tests

Several types of imaging tests are used to diagnose colorectal cancer.

CT scan

Computed tomography (CT) scans can be used in a few ways to help detect colorectal cancer, find signs of cancer in other areas of the body, or determine how well cancer treatment is working. Scans of the chest, abdomen and pelvis are performed to determine whether colorectal cancer has spread to other parts of the body, such as the lungs, liver or other organs. The scans also may help doctors stage the cancer. CT scans are typically performed before and at various points throughout colorectal cancer treatment, to help gauge whether treatment is working.

If your care team suspects that cancer has spread to your lungs or liver, a CT scan may be used during a biopsy to help them visualize exactly where the cancer cells are located in order to remove them. This is called a CT-guided needle biopsy.

A specific type of CT scan that integrates X-rays as well, called a virtual colonoscopy or CT colonography, may be used to screen for colorectal cancer. This type of test requires the same preparation as a colonoscopy, but it doesn’t require sedation. It may miss smaller polyps, but these are also less likely to be cancerous. Virtual colonoscopies are relatively new technology, but they may help spare some people from having unnecessary colonoscopies. If a virtual colonoscopy shows signs of cancer, additional tests will be done to help make a diagnosis and/or remove polyps.


Magnetic resonance imaging (MRI) may help doctors stage rectal cancer. MRIs use strong magnetic fields and radio waves to produce exceptionally detailed images. These tests also allow for greater soft-tissue contrast than a CT scan. The technician will likely give you a small amount of dye by injection or pill before the MRI. This dye will travel through your body temporarily to help the MRI pictures come out clearly. You’ll lie down in an MRI machine to have these images taken. MRIs can be used to see whether cancer has spread to other parts of the body, such as the brain or liver.

MRIs may also be used to look at the pelvis to see whether the initial colorectal tumor has grown into nearby structures or tissue. To get clearer pictures, your care team may do an endorectal MRI specifically. During an endorectal MRI, your doctor will insert a narrow probe into the anus in order to get high-quality images of rectal tissue.

PET/CT scan

A positron emission tomography-computed tomography (PET/CT) scan may be used to determine whether the colorectal cancer has spread to lymph nodes or other areas of the body, such as the liver or lungs. It also aids in staging the disease. It isn’t always used in diagnosing colorectal cancer, but your care team may recommend it in specific instances.

This test helps pinpoint a cancer’s location by highlighting the cells that use up energy the fastest—these tend to be cancerous cells. Before the test, a technician will inject you with a small, safe amount of radioactive sugar, which is the energy that cancerous cells will quickly absorb. The PET-CT scan then shows where the radioactive sugar was absorbed.

Chest X-ray

A chest X-ray may be done to scan your chest for signs that cancer has spread to the lungs, but other tests that produce three-dimensional images—like a CT scan—are likely to provide more useful images.


For tumors that may have spread to other organs, such as the liver, an angiography can help your care team visualize the blood vessels that are supplying nutrients to cancer cells. A dye is injected into an artery, and then X-ray pictures are taken of your blood vessels. This may help your care team determine whether the tumor can be removed and, if so, the best way to remove it.


This procedure may be used to produce images of internal organs from high-energy sound waves and echoes. Our doctors use ultrasound technology to check for tumors in abdominal organs, such as the liver, gallbladder and pancreas, especially if fluid has been found in the abdomen.

Ultrasounds may be performed in a few ways. Each one gives your care team a different set of information depending on how they’re done.

  • An endorectal ultrasound is done from within the rectum. It can tell your cancer care team how deep into the rectal wall the cancer cells have grown and whether a tumor is affecting nearby organs or lymph nodes. However, it cannot reliably detect cancer beyond the immediate area, such as the nearby lymph nodes.
  • An abdominal ultrasound is done over your abdomen. It can tell your cancer care team if the cancer has spread to organs in your abdomen, such as your liver or pancreas, but it won’t show the initial tumor in your colon or rectum.
  • An intraoperative ultrasound is done on the surface of the liver during surgery. This allows your care team to check the liver thoroughly to see whether cancer cells have grown there. If they find cancer cells in your liver, your care team may remove them during that same procedure.

Barium enema

A barium enema is used to take X-rays of the large intestine, which includes the colon and rectum. It helps doctors diagnose and stage colorectal cancer, and in some cases, it is used when a colonoscopy is not an option.

In this procedure, a doctor delivers an enema containing barium through a thin tube that is inserted through the rectum. The solution travels through the rectum and colon, coating the organs. Air is then released through the tube to help the colon expand and make it easier for your doctor to see abnormal growths. A series of X-rays are then taken to reveal images of the colon and rectum. These may enable the doctor to detect polyps and other suspicious tissues that need to be examined more closely or that should be removed during a colonoscopy.

Next topic: What are the colonoscopy and endoscopic procedures for colorectal cancer?

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