How Colon Cancer Is Diagnosed

Screening for colon cancer, which is generally recommended for all people age 45 and older (and perhaps earlier, if someone is at increased risk), can indicate a problem, but it cannot officially diagnose the disease. Instead, a diagnostic (rather than a screening) colonoscopy, biopsy, and imaging tests are needed to confirm and define the extent of colon cancer. While many people start this process because of routine recommended checks, others do so because of worrisome symptoms, an abnormal physical examination, or a new finding on a lab test (for example, iron deficiency anemia).

colon cancer diagnosis

History and Physical Exam

After reviewing any symptoms that may be suggestive of colon cancer (for example, blood in your stool or a change in bowel habits), as well as your risk factors for developing colon cancer (for example, a history of colon polyps and/or colon cancer), your doctor will perform a physical examination.

During the physical exam, your doctor will press on your abdomen to see if there is any discomfort or masses present. Your doctor will also check your vitals and skin to see if there is evidence of anemia (for example, a fast heart rate and/or paleness).

They also may perform a digital rectal exam (DRE), which involves inserting a gloved and lubricated finger into the rectum to feel for masses.

Colon Cancer Doctor Discussion Guide

Get our printable guide for your next doctor's appointment to help you ask the right questions.

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Labs and Tests

After a medical history and physical examination, your doctor may order labs, especially if your symptoms and/or exam are suspicious for colon cancer (or another intestinal problem).

While labs cannot determine whether or not you have colon cancer, they can help a doctor get to the bottom of what is going on, or at least help paint the overall clinical picture.

Some labs your doctor may recommend include: 

  • Complete blood count (CBC): This lab can determine whether or not you have anemia (low red blood cell count), which is a potential complication of colon cancer due to bleeding from the tumor.
  • Liver function test (LFT): Since colon cancer may spread to the liver, your doctor will order this lab to see how well your liver is functioning. 
  • Tumor markers: Some colon cancer cells make markers that travel to the bloodstream. For example, carcinoembryonic antigen (CEA) is a tumor marker for colon cancer. However, an elevated CEA may be seen in noncancerous conditions, and it's not always elevated in early-stage colon cancer. This is why professional groups like the American Society of Clinical Oncology (ASCO) do not recommend tumor markers as a diagnostic test for colon cancer. That said, it's worthy to note that CEA levels are often monitored before and after colon cancer treatment.

Diagnostic Colonoscopy

If your physical exam and/or blood tests are worrisome for colon cancer, your doctor will recommend more tests, most commonly a diagnostic colonoscopy, which is the most accurate test for diagnosing colon cancer. 

A diagnostic colonoscopy will also be performed if a routine screening colonoscopy was abnormal, meaning it was suspicious for cancer, or if at-home stool tests (for example, the stool DNA test) came back abnormal.

Furthermore, as an aside, sometimes diagnostic colonoscopies are incomplete (not able to be adequately performed). In this case, a virtual colonoscopy may be used to diagnose colon cancer.

During a colonoscopy, a gastroenterologist—a doctor who specializes in treating diseases of the digestive tract—inserts a flexible tube (called a colonoscope) into your anus. You can watch on a video monitor as the camera is threaded through your rectum, all the way to the end of your colon. If you're thinking about how unpleasant that must be, take heart—you're sedated during the procedure. 


In addition, if a suspicious mass is seen in the colon, the doctor can take a tissue sample (called a biopsy). A pathologist can look at the tissue underneath a microscope to see if cancer cells are present. If cancer is found, more lab tests may be performed on the biopsied sample, like tests that look for gene changes in the cancer cells. Results of these tests can help oncologists, or "cancer doctors," determine what treatments may work best or not at all.


Once the diagnosis of colon cancer is determined, the extent of the disease spread (called the stage) is determined with imaging tests. After the cancer has been staged, a treatment plan can then be devised. The imaging tests often used include:

  • Chest X-ray
  • Computed tomography (CT) scan of the abdomen and pelvis (also known as a CT colonography or virtual colonoscopy)
  • Magnetic resonance imaging (MRI) of the liver
  • Positron emission tomography (PET) scan (not as commonly used) 

There are five stages of colon cancer (0–4) and, in general, the earlier the stage, the easier the cancer is to treat. 


To understand the basics of the stages, think of the colon as a hollow tube with five layers, the innermost layer (called the mucosa), the second layer (called the submucosa), a third muscular layer (called the muscularis propia), and the outermost layers (called the subserosa and serosa). 

colon cancer stage at diagnosis
Illustration by Verywell

Stage 0

Stage 0 colon cancer is the earliest stage possible and is also called carcinoma in situ ("carcinoma" refers to cancer and "in situ" means original position or place). Stage 0 cancer has not grown beyond the inner layer of the colon (the mucosa). 

Stage 1

Stage 1 colon cancer means that the tumor has grown through the mucosa into the submucosa, or even into the muscular layer (called the muscularis propia). 

Stage 2

Stage 2 colon cancer means one of the following scenarios:

  • Cancer has grown into the outermost layers of the colon, but not through them.
  • Cancer has grown through the outermost layer of the colon. 
  • Cancer has grown through the wall of the colon and attached to or grown into other nearby tissues or organs.
  • Cancer has grown through the mucosa into the submucosa and possibly the muscularis propia.

Stage 3

Stage 3 colon cancer means one of several things:

  • Cancer has grown into the submucosa layer and spread to four to six nearby lymph nodes. 
  • Cancer has grown into the outermost layers and spread to one to three nearby lymph nodes or into areas of fat near the lymph nodes.
  • Cancer has grown into the muscularis propia, or the outermost layer of the colon, and spread to four to six nearby lymph nodes.
  • Cancer has grown into the submucosa and possibly into the muscularis propia, and spread to seven or more nearby lymph nodes.
  • Cancer has grown through the wall of the colon and has spread to four to six nearby lymph nodes.
  • Cancer has grown into the outermost layers of the colon and spread to seven or more nearby lymph nodes.
  • Cancer has grown through the wall of the colon, is attached to or has grown into nearby tissues or organs, and has spread to at least one nearby lymph node or into areas of fat near the lymph node.

Stage 4

Like stage 2 and 3 colon cancer, there are a number of different scenarios that describe stage 4 cancer. The bottom line, though, is that stage 4 colon cancer is synonymous with metastatic colon cancer because it implies that the tumor has spread to one or more distant organs (for example, the liver or lungs), to a distant set of lymph nodes or to distant parts of the lining of the abdominal cavity (called the peritoneum).

Dealing with a diagnosis of stage 4 cancer can be a quite challenging process, both physically and mentally. For most people, stage 4 colon cancer is not curable, but there are usually treatment options available. 

Colorectal cancer

Survival Rate

Colon cancer survival rates hinge greatly on the stage of the disease. In essence, survival rates are used by doctors as a way to discuss a person's prognosis, which is the expected course of the disease. For example, when colon cancer is caught early before it has spread beyond the colon, the prognosis is very good. A complete cure is, of course, the goal.

Five-Year Survival Rate

You may have heard the term "five-year survival rate" in your doctor's office or from those who have been diagnosed. 

The five-year survival rate is the percentage of people diagnosed with cancer who are still alive at least five years after their initial diagnosis.

For early stage cancers that are treated with the expectation of a complete cure, the five-year survival rate is sometimes considered the point at which a person is "out of the woods." After the five-year mark, it may become less likely for cancer to return. Some research suggests that people need to be followed closely for up to 10 years after diagnosis. This longer follow-up is to ensure that any cancer recurrences are caught early.

According to the National Cancer Institute and American Cancer Society, the five-year survival rates for people living with colon cancer (based on stage) are as follows:

  • Stage 1: 92%
  • Stage 2: 63% to 87%
  • Stage 3: 53% to 69%
  • Stage 4: 11%

Keep in Mind

It is important to remember that survival rates are estimates. A five-year survival rate can never predict what will happen in any one person's case. Other things, such as how cancer responds to treatment and the genetics of the cancer cells, will affect the chances of survival.

In addition, to come up with five-year survival rates, health experts need to study people who were treated for their cancer at least five years ago. Colon cancer treatments have changed and continue to change very quickly, which can improve outcomes.

Some treatments that are used now, such as targeted therapies, weren't available five years ago. Also remember that survival rates can include people who were diagnosed with colon cancer, but who later died of non-cancer-related causes.

This means that the five-year survival rates will likely look worse than what your five-year survival rate actually is. Be sure to discuss what you learn about five-year survival rates with your doctor. He or she can help you understand how this information might apply to your specific situation.

Differential Diagnoses

It's important to remember that one or more of the symptoms that may have brought you in to see your doctor (for example, rectal bleeding or abdominal pain) very well may be caused by another medical problem besides colon cancer.

That said, any new symptom should be evaluated, so a proper diagnosis and treatment plan can be initiated promptly.

Examples of other medical conditions that may mimic that of colon cancer include:


Hemorrhoids are swollen veins in your anus or lower rectum that may cause painless bleeding during a bowel movement and/or discomfort in the anal area. 

Irritable Bowel Syndrome

Abdominal spasms and cramping are common in irritable bowel syndrome but are generally eased with defecation. 


Appendicitis refers to inflammation of the appendix, which is a finger-like structure that sticks out from your colon. Appendicitis causes sudden pain around the umbilicus that moves toward the right lower side of the abdomen. Often, a person has nausea and/or vomiting and a loss of appetite. 


Diverticulitis refers to inflammation of colon diverticulum (a pouch located in the colon wall). With diverticulitis, pain is often sudden, constant, and present in the left lower abdomen. Other associated symptoms include constipation, loss of appetite, nausea and/or vomiting. 

Infectious Colitis

Infectious colitis means that the colon is inflamed by an infection (for example, with the bacteria Clostridium difficile). This condition may cause diarrhea in addition to abdominal pain and a fever. 

Bladder Infection (Cystitis)

In addition to discomfort in the suprapubic region (the area located over your pubic bone), a person with cystitis may experience symptoms like increased frequency or hesitancy with urination or burning with urination.

Kidney Stone

A kidney stone often causes pain in the lower back that may radiate to the abdomen, in addition to blood in the urine.

Frequently Asked Questions

What tests can detect colon cancer?

Colon cancer is most often diagnosed with imaging tests or stool-based tests. Colonoscopy is the "gold standard" test and allows for suspicious tissue to be removed to check for cancer, but CT colography and flexible sigmoidoscopy (which only looks at part of the colon) are other options. There are lab tests that can analyze stool samples to look for blood or molecular markers of cancer.

Can a CT scan detect colon cancer?

Yes. Computed tomography (CT) imaging for the colon is called CT colography or, sometimes, a virtual colonoscopy. It may be used for someone who's unable to tolerate a traditional colonoscopy or as one element of the diagnostic process. Studies suggest CT colography isn't as effective as colonoscopy for detecting small polyps or very early stage colon cancer, although it may, in some cases, pick up on polyps situated behind folds of tissue. If a CT colography reveals the presence of polyps, they will need to be removed via a colonoscopy.

Can colon cancer show up on an ultrasound?

Ultrasound is mainly used to see if colon cancer has spread. Three types of ultrasound imaging may be used:

  • Abdominal ultrasound can detect the spread of cancer to the liver, gallbladder, or pancreas (but not the presence of tumors in the colon or rectum.
  • Endorectal ultrasound is performed by inserting the transducer (the thin instrument to picks up the sound waves) into the rectum to see if cancer has infiltrated the walls of the rectum and how far. It also can reveal if cancer has reached nearby organs or the lymph nodes.
  • Intraoperative ultrasound, which is done during surgery, can detect if colon cancer has spread to the liver.

How is colon cancer staged?

Among the criteria considered when staging colon cancer are how deep the tumor is, whether fat and fascia (a type of connective tissue) are affected, whether cancer cells have invaded surrounding tissue or metastasized to distant parts of the body, and whether nearby lymph nodes are involved. Bloodwork and imaging tests are used to answer these questions, which will allow a doctor to assign diagnosed colon cancer one of five stages from 0 to IV, with IV being the most advanced.

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