How is Colon Cancer Diagnosed?

Medically reviewed by: Liza M. Capiendo, MD
Last modified on December 21st, 2023

Colon cancer — also known as colorectal cancer — is the 4th most common cancer* in the US overall. It’s also the fourth deadliest cancer, after lung, breast (females), and prostate (males) cancers. In this article we discuss how is colon cancer diagnosed.

Since colon cancer is relatively common, it’s important to know how it’s diagnosed as well as the symptoms that should prompt you to seek immediate medical attention.

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How is colon cancer diagnosed?

The best way to diagnose colon cancer is to have a colonoscopy. This is a procedure where a long, thin tube with a tiny video camera on the end is inserted into your rectum and passed through your entire colon. Your doctor will be able to see any abnormal growths, such as polyps, and remove them for further testing.

A colonoscopy is usually done every 10 years starting at age 45, or sooner if you have a family history of colon cancer. You may also need to have one more frequently if you’ve already had cancerous polyps removed.

But not all patients need to undergo a colonoscopy when there’s suspicion for colorectal cancer. In some cases, your healthcare provider may perform a sigmoidoscopy instead, which is a similar procedure but uses the tube to examine only the lower part of your colon.

Patients who can undergo a sigmoidoscopy (instead of a colonoscopy) are those with mild, occasional blood in stools and:

  • Age < 40 years
  • No other red flags for colorectal cancer (more on this later)
  • No risk factors for colorectal cancer (more on this later)

What is the first test for colon cancer?

The first test for colon cancer depends on your signs and symptoms as well as your healthcare provider.

Although a colonoscopy is the gold-standard test for diagnosing colon cancer, it’s not always the first test performed — especially if you’re asymptomatic (don’t have any symptoms).

In asymptomatic patients, the first test for colon cancer may be a stool-based test, such as a fecal immunochemical test (FIT) or a stool DNA test.

A FIT is a test that looks for blood in your stool, which can be an early sign of colon cancer. It’s important to note that blood in your stool does not always mean you have colon cancer, as it can also be caused by other conditions, such as hemorrhoids.

On the other hand, a stool DNA test looks for abnormal DNA in your stool that could be caused by colon cancer.

If either of these stool-based tests comes back abnormal, you’ll likely need to have a colonoscopy to confirm the results.

In patients with symptoms suggestive of colon cancer, such as rectal bleeding, changes in bowel habits, or abdominal pain, the first test is usually a colonoscopy.

Other tests (or procedures) that may be used to diagnose colon cancer include:

  • Digital rectal examination (DRE): A DRE is an exam where your healthcare provider inserts a gloved, lubricated finger into your rectum to feel for any abnormal growths.
  • Double-contrast barium enema: This is a rarely performed test where X-ray images are taken of your colon after it’s filled with a chalky liquid that contains barium. It’s performed in patients who refuse (or are unable) to undergo a colonoscopy. For example, someone who can’t be given procedural sedation will not be able to have a colonoscopy. Procedural sedation refers to medication given to help you relax during a medical procedure.

But at the end of the day, it’s important to remember that the only way to definitively diagnose colon cancer is to have a biopsy. This is performed during a colonoscopy, where a tissue sample is taken from an abnormal growth and examined under a microscope.

All of the other tests described above may raise suspicion for colon cancer, but the only way to confirm the diagnosis is with a biopsy.

Can colon cancer be detected in blood?

No, colon cancer cannot be detected in the blood.

There are blood tests that can be used to monitor the response of colorectal cancer to treatment, but they can’t be used to diagnose it.

For example, carcinoembryonic antigen (CEA) is a protein that may be elevated in patients with colorectal cancer. However, it’s not specific for colon cancer and can also be elevated in other conditions, such as pancreatitis and hepatitis.

As a result, it’s not used to screen for or diagnose colon cancer. But once the diagnosis of cancer has been established via biopsy during a colonoscopy, CEA levels can be used to monitor how the disease behaves or detect a recurrence.

In addition, your healthcare provider may perform additional blood tests if they’re suspecting colorectal cancer. These include:

  • Complete blood count: This is a routine test that measures the levels of different blood cells in your body. It can be used to check for anemia, which is common in patients with colorectal cancer.
  • Liver function tests: These tests are used to check for liver damage or dysfunction. This is important because colorectal cancer can spread to the liver.

What are the warning signs for colorectal cancer?

Warning signs for colorectal cancer include:

  • Gastrointestinal bleeding
  • Changes in bowel habits (e.g., diarrhea, constipation, or narrow stools)
  • Abdominal pain
  • Weight loss
  • Fatigue
  • Unexplained anemia, especially in men over 50 years of age and women who have undergone menopause

If you experience any of these symptoms, it’s important to see your healthcare provider so they can determine the cause.

Keep in mind that these symptoms can also be caused by other, less serious conditions. For example, rectal bleeding is often caused by hemorrhoids.

But it’s still important to see your healthcare provider so they can rule out more serious causes, such as inflammatory bowel disease (IBD) and colorectal cancer.

What increases the risk for colorectal cancer?

Risk factors for colorectal cancer can be related to age, hereditary syndromes, associated health conditions, lifestyle, and diet.


Colorectal cancer is more common in people over 40. This is why all people over the age of 45 should undergo screening for colorectal cancer.

Hereditary syndromes

Hereditary syndromes mean that you’re born with a genetic predisposition to developing certain diseases.

For example, familial adenomatous polyposis (FAP) and Lynch syndrome are two hereditary syndromes that increase the risk of colorectal cancer.

If you have FAP, you’ll develop hundreds to thousands of polyps in your colon and rectum. These polyps have a high risk of turning into cancer. Similarly, Lynch syndrome also increases the risk of colon cancer, as well as other cancers, such as uterine cancer.

This means if you have a family member with either of these syndromes, you may be at increased risk for colorectal cancer and should talk to your healthcare provider about genetic testing and screening options.

Associated health conditions

Certain health conditions are also associated with an increased risk for colorectal cancer. One important example is inflammatory bowel disease. IBD refers to a group of conditions that cause inflammation in the digestive tract. The two types are Crohn’s disease and ulcerative colitis.

People with inflammatory bowel disease have a higher risk of developing colorectal cancer, especially if they’ve had the condition for many years.

Other health conditions that increase the risk for colorectal cancer include diabetes and endocarditis (inflammation of the heart).

Lifestyle and diet

Smoking and alcohol consumption are two lifestyle factors that can increase the risk of developing colorectal cancer.

In addition, a diet rich in processed meat and fat is associated with a higher risk of colon cancer. A low-fiber diet and obesity are also associated with colon cancer.

People with any of these risk factors should be evaluated by their healthcare providers, and it’s very likely that they will perform a colonoscopy as the first test to rule out colon cancer in these patients.

Colon cancer is diagnosed by a colonoscopy followed by a biopsy

Remember, the only way to definitively diagnose colon cancer is through a biopsy, which can only be done during a colonoscopy.

In addition to a colonoscopy, there are various tests and procedures that help a doctor diagnose colon cancer. These include a sigmoidoscopy, DRE, fecal blood testing, and barium studies.


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