How is colorectal cancer diagnosed?

A colorectal cancer diagnosis usually involves several steps. If symptoms, a physical exam, or a screening test suggests there may be cancer in the colon or rectum, your healthcare provider will recommend additional testing to find the cause.

Tests may include a colonoscopy, biopsy, blood tests, and imaging. A biopsy is needed to confirm that abnormal tissue contains cancer cells. After colorectal cancer is confirmed, other tests can determine how far the cancer has spread and identify biomarkers that may help guide treatment.

Not every patient needs every test listed on this page. The types of tests you receive will depend on your symptoms, previous screening results, medical history, and what your healthcare team learns during the diagnostic process.

What symptoms can lead to a colorectal cancer diagnosis?

Colorectal cancer does not always cause symptoms, especially in its early stages. Some cancers are first discovered through routine colorectal cancer screening.

When symptoms do occur, they can include blood in the stool, rectal bleeding, persistent abdominal pain, unexplained weight loss, fatigue, or changes in bowel habits such as persistent diarrhea or constipation.

These symptoms have many possible causes and are not automatically a sign of cancer. However, persistent or unexplained symptoms may lead your healthcare provider to recommend further evaluation.

What happens if colorectal cancer is suspected?

Your healthcare provider will usually begin by asking about your symptoms and medical history. They may ask about previous colon polyps, family members with colorectal cancer, and conditions that increase colorectal cancer risk.

These conditions can include inflammatory bowel disease, such as Crohn's disease or ulcerative colitis, and inherited conditions such as Lynch syndrome or familial adenomatous polyposis (FAP).

A physical examination may also be performed. Depending on your symptoms, this can include examining the abdomen or performing a digital rectal exam to check the anus and rectum for abnormalities.

If additional evaluation is needed, your healthcare provider may refer you to a gastroenterologist. Gastroenterologists specialize in diagnosing and treating conditions affecting the digestive system.

Colonoscopy and colorectal cancer diagnosis

A colonoscopy is one of the most important tests used when colorectal cancer is suspected. During the procedure, a doctor uses a thin, flexible tube with a camera to examine the entire colon and rectum.

A colonoscopy allows the doctor to look for tumors, colorectal polyps, inflammation, bleeding, and other abnormal areas. Certain polyps can be removed during the procedure.

If suspicious tissue is found, samples can be collected and sent to a laboratory for examination. When colonoscopy is performed because symptoms or another test suggest cancer may be present, it may be referred to as a diagnostic colonoscopy rather than a screening colonoscopy.

What is the difference between a screening and diagnostic colonoscopy?

A screening colonoscopy is performed in someone without symptoms as part of routine colorectal cancer screening. Its goal is to find precancerous polyps or cancer before symptoms develop.

A diagnostic colonoscopy is performed to investigate a specific concern, such as rectal bleeding, iron-deficiency anemia, persistent bowel changes, or abnormal results from another screening test.

The procedure itself is similar, but the reason for performing it is different.

Biopsy: the test that confirms colon cancer

A biopsy is needed to confirm a diagnosis of colorectal cancer. During a biopsy, a small sample of suspicious tissue is removed so it can be examined under a microscope.

In many cases, the biopsy is performed during a colonoscopy. Small instruments can be passed through the colonoscope to collect tumor tissue from a suspicious area.

A pathologist, a doctor who specializes in examining cells and tissues, studies the sample to determine whether cancer cells are present.

What does pathology tell you about cancer cells?

The pathology report provides important information about the tissue collected during the biopsy. If cancer is present, the pathologist identifies the type of cancer and characteristics of the cancer cells.

Most colorectal cancers are adenocarcinomas, a type of carcinoma that begins in gland-forming cells lining the colon or rectum.

Pathology provides the accurate diagnosis needed before doctors can determine the most appropriate next steps. Additional testing of the tumor may also reveal molecular characteristics that influence treatment.

Stool tests and colorectal cancer screening

Stool tests are primarily used for colorectal cancer screening rather than to confirm a cancer diagnosis. These tests look for blood or abnormal genetic material that could be associated with cancer or certain polyps.

A fecal immunochemical test (FIT) uses a small stool sample to look for hidden blood. Another test, the fecal occult blood test, can also detect blood that cannot be seen with the naked eye.

Some stool-based screening tests combine an immunochemical test with a stool DNA test that looks for certain DNA changes associated with colorectal cancer or advanced polyps.

These tests can usually be completed at home using a test kit to collect one or more stool samples. An abnormal stool test does not diagnose colorectal cancer. A colonoscopy is generally needed to investigate the cause.

Flexible sigmoidoscopy and the colon and rectum

Flexible sigmoidoscopy is another procedure that can be used to look for abnormalities in the lower part of the colon and rectum.

During a sigmoidoscopy, a thin, flexible tube with a camera is inserted through the rectum. Unlike colonoscopy, flexible sigmoidoscopy does not examine the entire colon.

If polyps or suspicious areas are found, additional testing may be necessary. Depending on the findings, a full colonoscopy may be recommended to evaluate the rest of the colon.

CT colonography and colon cancer screening

CT colonography, sometimes called a virtual colonoscopy, uses computed tomography to create detailed images of the colon and rectum.

It is one of several colorectal cancer screening tests available for certain people. Unlike a traditional colonoscopy, CT colonography does not allow the doctor to remove polyps or take a biopsy during the test.

If CT colonography identifies a suspicious mass or significant polyp, a colonoscopy is generally needed for further evaluation and tissue sampling.

What happens after an abnormal colorectal cancer screening test?

Several screening tests can identify findings that need additional evaluation. These include stool-based tests, colonoscopy, flexible sigmoidoscopy, and CT colonography.

If a stool test, flexible sigmoidoscopy, CT colonography, or certain other screening tests produce abnormal results, your healthcare provider may recommend a colonoscopy.

Screening and diagnosis are therefore related but different. Screening looks for possible cancer or precancerous changes in people who do not have symptoms. Diagnostic testing determines whether an abnormal finding or symptom is actually caused by cancer.

Imaging tests after a colorectal cancer diagnosis

Once colorectal cancer has been diagnosed, imaging tests can help determine whether the disease has spread beyond the colon or rectum. This process is called staging.

A computed tomography (CT) scan of the chest, abdomen, and pelvis is commonly used to look for cancer in areas such as the liver, lungs, and lymph nodes.

Magnetic resonance imaging (MRI) is especially important for rectal cancer because it can provide detailed information about how deeply a tumor has grown and whether nearby structures or lymph nodes appear involved.

PET scans are not routinely needed for every patient but may be useful in selected situations. Ultrasound, including endorectal ultrasound, may also be used in certain patients with rectal cancer.

The imaging tests recommended for you will depend on the location and characteristics of your cancer.

Blood tests used in colorectal cancer diagnosis and treatment

Blood tests cannot confirm colorectal cancer on their own, but they can provide important information before and after a diagnosis.

A complete blood count (CBC) measures red and white blood cells and platelets. A low red blood cell count, or anemia, can sometimes result from slow bleeding caused by a colorectal tumor.

Blood chemistry and liver function tests can help healthcare providers evaluate your overall health and look for findings that may require additional investigation.

Blood tests may also be repeated during treatment to monitor how your body is responding to surgery, chemotherapy, or other treatments.

What is the CEA tumor marker?

Carcinoembryonic antigen, or CEA, is one of the tumor markers commonly associated with colorectal cancer.

Some colorectal cancers release higher amounts of CEA into the blood. However, CEA is not accurate enough to diagnose colorectal cancer by itself, and some people with colorectal cancer have normal CEA levels.

CEA is more useful after diagnosis. Doctors may measure it before treatment and then monitor it over time to help assess treatment response or look for signs that cancer has returned.

Biomarker testing of colorectal cancer cells

Biomarker testing looks for genes, proteins, and other features within cancer cells. It may also be called molecular testing or tumor profiling.

These results can help doctors understand the biology of a person's cancer and determine whether certain targeted therapies or immunotherapies may be appropriate.

Biomarker testing becomes especially important when treatment decisions are being made for advanced or metastatic colorectal cancer.

KRAS, NRAS and BRAF testing

KRAS, NRAS, and BRAF are genes involved in pathways that control cell growth. Changes, or mutations, in these genes can affect which treatments are likely to work.

For example, certain KRAS or NRAS mutations can predict that particular EGFR-targeted therapies are unlikely to be effective.

BRAF testing can identify tumors with a BRAF V600E mutation, which can influence both prognosis and treatment options.

MSI and mismatch repair testing

Colorectal tumors are commonly tested for microsatellite instability (MSI) or deficient mismatch repair (dMMR).

Tumors that are MSI-high (MSI-H) or dMMR may respond particularly well to certain immunotherapies. These results can therefore directly influence treatment.

MSI or mismatch repair testing can also help identify patients who may have Lynch syndrome, an inherited cancer syndrome associated with colorectal cancer and several other cancers.

HER2 and NTRK testing

Some colorectal cancers have changes involving HER2 or NTRK that can potentially be targeted with specific treatments.

HER2 testing looks for increased HER2 activity in the tumor. NTRK testing looks for rare gene fusions that can drive cancer growth.

Your oncology team will determine which biomarkers should be tested based on the characteristics and stage of your cancer.

How does diagnosis affect colorectal cancer treatment and prognosis?

After colorectal cancer is confirmed, your healthcare team combines the biopsy, pathology, imaging, blood test, and biomarker results to understand the cancer as completely as possible.

The stage of the cancer is one of the most important factors affecting prognosis and treatment. Treatment may include surgery, chemotherapy, radiation therapy for some rectal cancers, targeted therapy, immunotherapy, or a combination of approaches.

Tumor biomarker results can further personalize treatment by identifying therapies that are more or less likely to work for a particular cancer.

For some patients, clinical trials may provide additional treatment options. Your care team can help determine whether a clinical trial is appropriate for your diagnosis.

What tests are used for colon cancer versus rectal cancer?

Many diagnostic tests are the same for colon cancer and rectal cancer. Both generally require colonoscopy and biopsy to establish the diagnosis.

There are some differences after diagnosis. MRI of the pelvis plays an especially important role in evaluating rectal cancer because doctors need detailed information about the tumor's relationship to the rectal wall, lymph nodes, and nearby structures when planning treatment.

It is also important to distinguish colorectal cancer from anal cancer, which begins in a different part of the digestive tract and is diagnosed, staged, and treated differently.

Which colorectal cancer tests will I need?

You probably will not need every test described on this page. The tests your healthcare team recommends depend on how the cancer was discovered, where it is located, its stage, your medical history, and the information needed to plan treatment.

Some tests answer different questions. Colonoscopy finds abnormal areas and allows tissue to be collected. Biopsy confirms cancer. Imaging determines where the cancer is located and whether it has spread. Blood tests provide information about your overall health, while biomarker testing can help guide treatment.

Ask your healthcare team what each test is looking for and how the results affect your diagnosis or treatment plan. Understanding why a test is being performed can make the diagnostic process easier to navigate.