Understanding Colorectal Cancer

How is Colorectal Cancer Diagnosed?

This is the sixth page in the Understanding Colorectal Cancer Guide. This guide was developed by the HealthTree Education Team and was last updated and reviewed on April 3rd, 2026. 

If your doctor thinks you may have colorectal cancer, there are several tests and procedures that can confirm a diagnosis. A single test is usually not enough. Diagnosis relies on a combination of your symptoms, a physical exam, and test results.

This page lists all of the different tests that may be needed. Some are used to confirm a colorectal cancer diagnosis. Others are used to learn more about the cancer, such as genetic changes, that can help guide treatment. Some of these tests may also be used to monitor how well treatment is working. You may not have all of the tests on this list.

Talk to your care team if you have questions about which tests you received and what the results mean for your diagnosis.

Biopsy

A biopsy is the only way to confirm a diagnosis of colorectal cancer. During a biopsy, a small sample of tissue is removed from the colon or rectum and examined under a microscope. In many cases, a biopsy is done during a colonoscopy. 

The doctor uses small tools passed through the colonoscope to take a tissue sample from any suspicious areas or polyps. A pathologist (a doctor who specializes in examining tissue) then analyzes the sample to determine if cancer cells are present.

Imaging tests to diagnose colorectal cancer

Imaging tests create pictures of the inside of the body and help doctors see whether cancer has spread beyond the colon or rectum:

• Computed tomography (CT). A CT scan is the most common imaging test used to check whether colorectal cancer has spread to the liver, lungs, or lymph nodes. A CT colonography (virtual colonoscopy) can also be used to look for polyps or tumors in the colon.
• Magnetic resonance imaging (MRI). An MRI is especially useful for rectal cancer, to see how deep the tumor has grown into the rectal wall and whether nearby lymph nodes are affected.
• Positron emission tomography (PET). A PET scan is sometimes used to look for cancer spread throughout the body
• Chest X-ray. This may be used to check whether cancer has spread to the lungs.
• Ultrasound. An endoscopic ultrasound (EUS) can help evaluate the depth of a rectal tumor

Blood tests to diagnose colorectal cancer

Blood tests used in the diagnosis and monitoring of colorectal cancer include:

• Complete blood count (CBC). A CBC checks for anemia (low red blood cells), which can be a sign of bleeding from a tumor
• Liver function tests. This test helps detect whether cancer may have spread to the liver
• Carcinoembryonic antigen (CEA). CEA is a protein that may be elevated in people with colorectal cancer. It is most useful for monitoring treatment response and checking for recurrence rather than for initial diagnosis

Biomarker and genetic tests to diagnose colorectal cancer

Biomarker testing looks for specific genes, proteins, or other features of the cancer cells. These can help guide treatment decisions. This is sometimes called molecular testing or tumor profiling. For colorectal cancer, important tests include:

• KRAS, NRAS, and BRAF mutations. Mutations in these genes affect which targeted therapies are likely to work
• HER2 testing. This test checks for overexpression of the HER2 protein, which may be targeted by certain therapies.
• Microsatellite instability (MSI)/mismatch repair deficiency (dMMR). Cancers with high MSI (MSI-H) or dMMR respond well to immunotherapy. MSI testing is also used to screen for Lynch syndrome.
• NTRK gene fusions. This is a rare change that may be targetable with specific drugs.