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Is a Colonoscopy the Best Test For Colon Cancer?

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    Is a Colonoscopy the Best Test For Colon Cancer?

    Colorectal cancer often develops without signs or symptoms and, if not caught during its early stages, can spread throughout the body and ultimately lead to death. Colorectal cancer begins with abnormal tissue growths known as polyps, which grow in the lining of the large intestine and can become cancerous with time if not removed. These growths eventually grow into large polyps or tumors that can spread their cancerous cells to nearby tissues and organs. Detection and removal of polyps is proven to halt the development of colorectal cancer. 

    Consequently, many clinicians and experts believe that the best way to detect colorectal cancer is by a colonoscopy – the examination of the entire colon using a very small scope that can capture images of the lining of the intestine where the polyps develop. 

    A colonoscopy gives physicians the opportunity to examine the entire colon to detect the presence of polyps and, if found, remove them immediately. While other colorectal screening tests can detect colorectal cancer and the presence of polyps, only a colonoscopy offers the physician a chance to immediately remove precancerous abnormal tissues during the procedure.

    Colonoscopies: The Gold Standard for Colon Cancer Screenings

     The reason so many experts agree that a colonoscopy is the best test for colon cancer is because of its high degree of sensitivity and specificity.  

    Sensitivity: Indicates the likelihood that the test will detect if a condition or disease is present in a patient. A test with a high sensitivity is less likely to give a patient a false negative result. 

    Specificity: Indicates the likelihood that the test will rule out if a condition or disease is present. A test with a high specificity will give a low number of false positive results.

    These two factors help to describe how valid or accurate a test is. Colonoscopies are highly sensitive and specific. This means they correctly identify people who have polyps or colon cancer present and accurately identify people who do not have polyps or colon cancer. 

    In the medical world, a screening test will be considered the “gold standard” if it is the most accurate test for detecting the presence of a disease or condition. Physicians and medical experts consider colonoscopies the gold standard for detecting colon cancer due to for the following reasons:

    • A colonoscopy is highly sensitive (94%) and specific (73%). 
    • A colonoscopy examines the colon in its entirety as opposed to other tests that only examine sections (flexible sigmoidoscopy).
    • Polyps can be removed during the colonoscopy when detected. If polyps are detected during a stool test, a patient still needs a colonoscopy to remove precancerous growths. 
    • The test typically is repeated every ten years due to the thorough exploration of the colon. 
    • Colonoscopies definitively identify sources of rectal bleeding during the procedure. 
    • Stool tests have a significant percentage of results that return as false positive due to the number of conditions that can cause bleeding (for example, peptic ulcers, ulcerative colitis, gastroesophageal reflux disease, and hemorrhoids). 
    • Patients with any positive result from a stool test or visual colonoscopy will need a colonoscopy to remove abnormal tissue growths. 

    Multiple studies show that mortality due to colorectal cancer is 68% – 88% lower in people who have undergone screening colonoscopies over those who have not. 

    How Effective Is a Colonoscopy At Diagnosing Colorectal Cancer?

    In the past, physicians believed that a colonoscopy was able to detect colorectal cancer 100% of the time. A colonoscopy is highly effective for finding polyps and cancerous tissues, but not at all times. 

    • Colonoscopies detect 94% of all colorectal cancer cells
    • Patients who are older, have a family history of colon cancer or polyps, and have a previous colonoscopy where a polyp are at risk for these missed cases of detecting colon cancer
    • Patients at a low risk for colon cancer have a nearly 98% chance that colon cancer will be found during a colonoscopy
    • Physicians who take more time during a colonoscopy have a higher rating for detecting colorectal cancer or polyps than those who rush the procedure
    • Patients who follow the dietary and medication restrictions and perform a complete bowel cleanse are less likely to have a missed colon cancer than those who do not follow the directions

    Alternatives to Colonoscopy, and How Do They Compare Accuracy?

    Fecal Occult Blood Tests (FOBT)

    Summary: Fecal occult blood tests detect microscopic quantities of blood using a chemical reaction. Because the test uses a chemical to detect the presence of blood, many dietary sources can mimic blood such as red meat, antioxidants like vitamin C, and iron supplements. A person taking this test needs to avoid eating these foods several days before the stool sample is taken. The test is widely available, inexpensive, and easy to take. 

    Effectiveness: Studies have shown that an annual FOBT reduced the mortality rate from colorectal cancer 33%. The sensitivity rate of detecting cancer approximates 50%. 

    Fecal Immunochemical Test (FIT)

    Summary: The FIT test also detects microscopic quantities of blood but does not require a person to stop eating red meats or supplements. It is easier to take than the FOBT test, requires less samples, and has a higher sensitivity for detecting cancerous adenomas (abnormal tissue growths). 

    Effectiveness: Compared to a colonoscopy with a sensitivity rate of 98%, the FIT test has a 79% sensitivity rating with 94% specificity for detecting colorectal cancer. Studies show that regular FIT screenings show a 22% reduction in the incidence of colorectal cancer. 

    VIrtual Colonoscopy (CT Colonography)

    Summary:  A virtual colonoscopy uses radiology to detect polyps or abnormal areas of the rectum or colon without sedation or an internal examination. The test does require the patient to cleanse the bowels the day before the procedure. 

    Effectiveness: The sensitivity and specificity of a virtual colonoscopy for detecting polyps in patients with no presenting symptoms are 66.8% and 88.3% respectively. A colonoscopy has a sensitivity rating of 92.5% -96% and 73.2% respectively. 

    Stool DNA Testing (Cologuard®)

    Summary: Stool DNA colorectal screening tests detect abnormal DNA present in stool samples that indicate the presence of colon cancer. 

    Effectiveness: Cologuard® is highly sensitive (92%) for detecting colon cancer but rates poorly for finding large adenomas (42%). 

    Sigmoidoscopy

    Summary: Both a sigmoidoscopy and a colonoscopy use a scope to examine the colon without the need for a surgical intervention. A sigmoidoscopy only examines the lower portion of the large intestine called the sigmoid (S-curve) and the rectum for polyps and abnormal tissues. Like a colonoscopy, a sigmoidoscopy allows the examining physician to remove polyps found in the lower region of the colon and take a biopsy to test for cancerous cells. 

    Effectiveness: Because only the lower portion is visualized, a sigmoidoscopy will detect less cancers that are higher up in the lower intestinal tract. The sensitivity and specificity for detecting colorectal cancer and abnormal adenomas averages 67%.When compared to the colonoscopy sensitivity and specificity ratings of 94% and 73% respectively, a colonoscopy is better at detecting colon cancer. 

    References

    Begley, S.(2016, March 14). Is colonoscopy the gold standard for colorectal cancer screening? Retrieved from Stat: https://www.statnews.com/2016/03/14/colonoscopy-colon-cancer-screening/

    Issa, I. A., & Noureddine, M. (2017, July 28). Colorectal cancer screening: An updated review of the available options. World Journal of Gastroenterology, 23(28), 5086-5096. Retrieved from PMC: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5537177/

    Niedermaier, T., Weigl, K., Hoffmeister, M., & Brenner, H. (2017, June). Diagnostic performance of flexible sigmoidoscopy combined with fecal immunochemical test in colorectal cancer screening: meta-analysis and modeling. European Journal of Epidemiology, 32(6), 481-493. Retrieved from PubMed: https://pubmed.ncbi.nlm.nih.gov/28667446/

    Swift, A., Heale, R., &, Twycross, A. (2019) What are sensitivity and specificity? Evidence-Based Nursing, 23(1). Retrieved from EBN: https://ebn.bmj.com/content/23/1/2

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