Diagnosis of Colorectal Cancer

Executive Summary

Different tests are available to diagnose colorectal cancer, which depends on signs and symptoms, age and health status, types of cancer suspected, and earlier medical tests. Biopsy and imaging tests are the common diagnostic approach for colorectal cancer. Other diagnostic tests for colorectal cancer include colonoscopy, biopsy, biomarker testing of tumor, blood tests, Computed tomography (CT or CAT) scan, Magnetic Resonance Imaging (MRI), Positron emission tomography or PET-CT scan, ultrasound, and chest x-ray.

Diagnosis of Colorectal Cancer

Many tests are performed to find or diagnose colorectal cancer. They also do tests to see if cancer has spread to another part of the body from where it began. If this happens, it is called metastasis. For instance, imaging tests can represent if cancer has spread. Imaging tests show pictures of the body from the inside. Doctors can also do tests to learn which treatments would work best.

A biopsy is the sure short way for the doctor to know if an area of the body has cancer for most types of cancer. In the biopsy, the doctor takes a small tissue sample for testing in a laboratory. The doctor may suggest other tests if a biopsy cannot help with diagnosis of colorectal cancer.

The doctor may consider given factors when choosing a diagnostic test:

  • Your signs and symptoms
  • The age and general health status
  • The type of cancer suspected
  • The result of earlier medical tests

In addition to a physical examination, the below-mentioned tests may be used for diagnosis of colorectal cancer ​1​:

  • Colonoscopy – A colonoscopy helps the doctor see inside the whole rectum and colon when a patient is sedated. A flexible, lighted tube known as a colonoscope is inserted into the rectum and the entire colon to look for polyps/ cancer. A doctor can eliminate polyps or other tissue during this process for examination. The removal of polyps can also help in the prevention of colorectal cancer.
  • Biopsy – A biopsy removes a small part of tissue to examine under a microscope. A biopsy can also be done as surgery to remove the entire tumour. The type of biopsy performed depends upon the position of the tumour. 

A biopsy can be done during the colonoscopy or on any tissue removed during surgery. Occasionally, a CT scan or ultrasound performs a needle biopsy. A needle biopsy removes tissue through the skin using a needle guided into the tumour.

  • Biomarker testing of tumour – The doctor may recommend running laboratory tests to identify proteins, specific genes, and factors unique to the tumour. 

All colorectal cancers should be tested for mismatch repair proteins, called a mismatch repair defect (dMMR). There are two reasons for this particular testing. First, it is a way to check for Lynch syndrome. Second, the results will be used to determine if immunotherapy can be considered in patients with metastatic cancer. This testing can either be done using special staining of the tissue taken from a biopsy or surgery or by doing analyses that search for changes known as microsatellite instability (MSI) ​2​.

If you have recurrent or metastatic colorectal cancer, a sample of tissue from that area where it spread or recurred is preferred for testing, if available.

  • Blood Tests – As colorectal cancer usually bleeds into the large intestine or rectum, people with the disease can become anemic. A test of the count of red cells in the blood, which is part of a complete blood count (CBC), can indicate if bleeding is occurring ​3​.

Another blood test detects the levels of a specific protein called carcinoembryonic antigen (CEA) ​1​. High levels of CEA may indicate that cancer has spread to other parts of the body. CEA is not the perfect test for colorectal cancer because levels are high for only around 60% of people with colorectal cancer that has spread to organs than the colon. Additionally, other medical conditions can cause an increase in CEA. A CEA test is mainly used to monitor colorectal cancer for people already receiving treatment. It is not helpful as a screening test. 

  • Computed tomography (CT or CAT) scan – A CT scan clicks pictures of the body from inside using x-rays taken from different angles. A computer combines photos into a detailed, 3-dimensional image that shows abnormalities or tumors. A CT scan helps measure the tumor’s size. Sometimes, a special dye known as a contrast medium is given before the scan to provide better detail on the image. This dye can be delivered into a colorectal cancer patient’s vein or given as a pill or liquid to swallow.
  • Magnetic resonance imaging (MRI) – An MRI uses magnetic fields, not x-rays, to produce detailed body images. A specific dye known as contrast medium is given before the scan to create a better picture. This dye can be delivered into a patient’s vein or given as a pill or liquid to swallow.
  • Positron emission tomography or PET-CT scan – PET scan creates images of organs and tissues present inside the body. A small quantity of a radioactive substance is injected into the patient’s body which is taken up by cells using the most energy. The cancer cells which use energy actively take up the radioactive substance, and the scanner then spots this substance to produce images of the inside of the body. In bone cancer, this scan provides a more comprehensive view to determine the presence of abnormal activity, even before a tumor may have developed.       
  • Ultrasound – An ultrasound uses sound waves to create a picture of the internal organs to determine if cancer has spread. Endorectal ultrasound is commonly used to determine how deeply rectal cancer has grown and help plan treatment. However, this test cannot correctly detect colorectal cancer that has spread to surrounding lymph nodes or beyond the pelvis. Ultrasound can also be used to look through the liver, although CT scans or MRIs are better for finding tumors in the liver. 
  • Chest X-ray – An x-ray is a method to create a picture of the structures inside of the body, using a small quantity of radiation. A chest x-ray can help doctors determine if cancer has spread to the lungs.

When diagnostic tests are completed, the doctor will review the results with you. These results help the doctor describe cancer if the diagnosis is colorectal cancer.


  1. 1.
    Świderska M, Choromańska B, Dąbrowska E, et al. Review The diagnostics of colorectal cancer. wo. Published online 2014:1-6. doi:10.5114/wo.2013.39995
  2. 2.
    Rawson JB, Bapat B. Epigenetic biomarkers in colorectal cancer diagnostics. Expert Review of Molecular Diagnostics. Published online June 2012:499-509. doi:10.1586/erm.12.39
  3. 3.
    Levin B, Lieberman DA, McFarland B, et al. Screening and Surveillance for the Early Detection of Colorectal Cancer and Adenomatous Polyps, 2008: A Joint Guideline from the American Cancer Society, the US Multi-Society Task Force on Colorectal Cancer, and the American College of Radiology. CA: A Cancer Journal for Clinicians. Published online May 1, 2008:130-160. doi:10.3322/ca.2007.0018