Why is a Colonoscopy Performed?
If you are over the age of 50, be prepared for your doctor to ask you to get a colonoscopy to screen for colorectal cancer (CRC) – the second leading cause of cancer deaths in the United States.Your doctor’s insistence in ordering this procedure is due to the fact that colonoscopies have been shown to significantly reduce the incidence and mortality of colorectal cancer since being put into practice.
Colonoscopy screening has helped to decrease the mortality rate of CRC-related deaths by 30% in the last ten years. Colonoscopies are also associated with at least a 60% reduction in colorectal cancer incidence when used as a diagnostic or screening procedure.
During a colonoscopy, a gastroenterologist uses a tube equipped with a camera on one end to closely examine the lining of the rectum and colon for signs of polyps, abnormal growths that can become cancerous with time. Colonoscopies allow physicians to detect colorectal cancer early before symptoms develop, thus greatly improving a patient’s ability to fully recover from cancer before it spreads.
Colonoscopies are the primary method that doctors use to diagnose chronic gastrointestinal diseases and help patients on the path to recovery. If you are suffering from abdominal or rectal bleeding, chronic diarrhea, constipation, or weight loss, your doctor may order a colonoscopy to diagnose the cause. Many people are unaware that they are anemic because of intestinal lesions that slowly bleed leading to a condition known as iron-deficient anemia. Other patients visit their healthcare provider with gastrointestinal symptoms of extreme pain and discomfort and want to understand the cause and receive treatment.
In this article, you will learn what medical conditions a colonoscopy can diagnose, how accurate and effective they are in diagnosis, and how the procedure compares to home cancer screening methods.
Why is a Colonoscopy Performed?
During a colonoscopy, a physician uses a long and narrow flexible tube equipped with a camera on one end to closely examine the lining of the colon and rectum.Colonoscopies can be ordered by your doctor as a diagnostic or preventative measure. A screening colonoscopy is usually ordered by your doctor if you have no presenting gastrointestinal symptoms before the procedure nor history of polyps or colorectal cancer. A diagnostic colonoscopy is ordered if you or a family member has a history of colorectal cancer or you are suffering from gastrointestinal symptoms. Before you go in for your colonoscopy, be sure to call your insurance provider to discuss your benefits and see if they cover both screening and diagnostic colonoscopies. Many insurance providers only cover a screening colonoscopy and will ask you to pay for the procedure partially or completely out-of-pocket.
A diagnostic colonoscopy is ordered for those patients with gastrointestinal symptoms to locate lesions and diagnose a disease or condition. This procedure helps doctors to diagnose inflammatory conditions such as ulcerative colitis and diverticulosis and to find the source of blood found in the stools or bleeding from the rectum. The criteria for a diagnostic colonoscopy includes:
- Family history of colorectal cancer or polyps
- Gastrointestinal symptoms such as blood in the stool, weight loss, and abdominal cramping
- Past history of polyps or colorectal cancer
- Past history of diverticulitis
A preventative colonoscopy is used to screen for colorectal cancer in patients over the age of 50 with no history of colon cancer or polyps. During a screening colonoscopy, your doctor will be looking for abnormal tissue growths known as polyps or adenomas that form on the lining of the colon or rectum. Polyps are usually benign, but over time can develop into cancerous tumors. The average amount of time it takes for a polyp to develop into invasive cancer is ten years. Because the progression of a precancerous tumor to cancer is slow, colonoscopies are highly effective in catching CRC in its earliest stages before symptoms or signs are present. The criteria for a screening colonoscopy includes:
- No family history of colorectal cancer or polyps
- No past history of colorectal cancer or polyps
- Absence of presenting symptoms such as weight loss, abdominal cramping, or blood in the stool
What Does a Colonoscopy Diagnose?
While many colonoscopies are performed to screen for colorectal cancer, there are various conditions and diseases that are diagnosed using this procedure. These include:
- Colorectal cancer
- Adenomas or precancerous tumors (polyps)
- Ulcerative colitis. An inflammatory bowel disease (IBD) of the colon and rectum
- Crohn’s disease. An IBD that causes inflammation of the digestive tract
- Diverticulosis. Caused by bulging pouches (diverticula) on the walls of the intestines
- Acute diverticulitis. Occurs when diverticula become infected
- Strictures. Narrowed sections of intestines that can be dilated using a balloon
How Accurate is a Colonoscopy?
Colorectal cancer (CRC) screening methods like colonoscopies are designed to detect cancer before any symptoms or signs arise. Important statistics to consider about the effectiveness of a colonoscopy in the fight against colorectal cancer are:
- Screening colonoscopies contributed to the number of CRC-related deaths being cut in half between the years 1970 through 2016
- Deaths from colorectal cancer have gone down 30% in the last ten years
- Following a colonoscopy, patients are at a 67% lower risk of death from colorectal cancer when compared to a person with no endoscopic screening
- Regular screenings every 10 years reduces risk of CRC-related death by 88%
Colonoscopies are considered the most accurate test for the early detection of cancer of the colon and rectum. Nonetheless, a significant number of cancers are missed and many people are diagnosed with CRC after a colonoscopy. Important statistics regarding the efficacy of colonoscopies include:
- 94% of colorectal cancers are caught during a colonoscopy
- Approximately 4%-6% of colorectal cancers develop between screenings or are missed during the procedure itself
- Several studies have shown that colorectal polyps are missed at a rate of between 6%-28%.
There are several reasons that contribute to these inaccuracies, including inadequate preparation of bowels before the procedure and poor technique on the part of the internal physician performing the procedure. If a patient has multiple polyps, it is important that all are removed completely to ensure no abnormal tissue is left behind that can turn cancerous in time. Many cases of cancer that develop between colonoscopies are attributed to an incomplete removal of an adenoma.
There are several risk factors that contribute to the development of CRC cancer after a colonoscopy including:
- Advanced age in men and women
- History of diverticulosis
- Prior abdominal or pelvic surgery
- Cancers found in the proximal colon
- Earlier-stage cancers
- Rapidly progressing cancers
- Higher number of adenoma growths
- Family history of colorectal cancer
- Incomplete colonoscopy
- Inadequate bowel preparation
- Incomplete removal of adenoma and/or polyp
More specifically, an important finding in a colonoscopy is the presence of an adenoma (benign tumor) or polyp. These adenomas or polyps may not appear easily on the camera due to small size and flat shape.
Size of polyps. Sensitivity for detecting adenomas 10 mm or larger ranges from 89% to 98%. This sensitivity drops for smaller polyps (less than 6mm) with a sensitivity rating of between 75% to 93%. In these cases, a proper bowel preparation and second colonoscopy clearing in one year can assist doctors in finding these smaller polyps.
Shape of polyps. Some adenomas or lesions are missed due to their flat shape.Flat lesions are lesions that do not protrude from the surface of the intestinal lining and require enhanced preparation to be identified with the camera. These adenomas are more likely to be malignant than the protruding type of adenoma. The combined use of an indigo carmine dye sprayed on the lining combined with the colonoscope (tiny camera used in procedure) assists the doctor in identifying these lesions.
Incomplete removal of polyps, inadequate procedural technique, and poor bowel preparation are the biggest contributors to missed malignancies and identification of adenoma
How effective are colonoscopies at diagnosis?
Your doctor will order a diagnostic colonoscopy if you are suffering from the following signs or symptoms:
- Rectal bleeding or pain
- Lower abdominal pain
- History of polyps or colorectal cancer
- Iron deficiency anemia (IDA)
- Unexplained changes in bowel movements
- Lower gastrointestinal bleeding
- Unexplained weight loss
- Obstruction of the colon
As a primary diagnostic test, a colonoscopy is highly effective in diagnosing gastrointestinal conditions, finding polyps, and identifying bleeding sites that are causing iron deficiency anemia. A doctor may suspect that a lesion in the intestinal wall is causing anemia and needs to diagnose the cause. Iron deficiency anemia can be caused by blood loss from intestinal lesions that may be malignant. In a colonoscopy, the surgeon can find these lesions and remove a small piece (biopsy) to test for cancer.
If you are suffering from chronic constipation or diarrhea, a colonoscopy can effectively diagnose the cause due to its ability to allow the gastroenterologist a view of the entire colon. People who have an acute lower gastrointestinal bleed may not have obvious bright red bleeding from the rectum but may present with “occult” bleeding. In these cases, a surgeon can use a colonoscopy to find the source of the bleed precisely and with fewer complications than the alternative, arteriography (which involves a tube placed in an artery through the neck).
Lastly, colonoscopies are considered the most effective method in diagnosing colorectal cancer. A routine colonoscopy may reveal a polyp that can be biopsied, tested for cancer, and removed during the procedure.
How does this compare to at-home cancer screening?
Home cancer screening methods are less invasive than colonoscopies, require no bowel preparation, and allow you to test in the comfort of your home. Three different home CRC cancer screening tests have been shown to have varying degrees of efficacy. These include:
Fecal immunochemical test (FIT). Detects cancer with an accuracy rating of 79% and a false positive results rating of 5%.
Guiac-based fecal occult blood test (gFOBT). While the gFOBT can detect microscopic amounts of blood in the stool, it is considered the least accurate test for identifying cancerous cells. The accuracy of this test ranges from 20% to 50% in detecting CRC cells.
Multitarget stool DNA test (FIT-DNA). Most effective in detecting cancer, the FIT-DNA test can detect cancer with a 92% accuracy. Unfortunately, 14% of tests are returned with a false-positive test which warrants a colonoscopy to rule out cancer. Cologuard (Exact Sciences) is the only at-home fecal occult blood test (FIT-DNA) that is approved by the FDA to detect the presence of cancerous cells as an alternative to a colonoscopy.
Ashtari, S., Pourhoseingholi, M. A., & Zali, R. M. (2016, November 17). Overview of diagnostic and treatment colonoscopy function in gastrointestinal diseases. Journal of Liver Research, Disorders, and Therapy, 2(4). Retrieved from MedCrave: https://medcraveonline.com/medcrave.org/index.php/JLRDT/article/view/13404/25223
Doubeni, C. A., et. al. (2018, September). Effectiveness of screening colonoscopy in reducing the risk of death from right and left colon cancer: a large community-based study. Gut, 67, 291-298. Retrieved from BMJ Open Access: https://gut.bmj.com/content/gutjnl/67/2/291.full.pdf
Lee, J. K., et., al. (2019) Long-term risk of colorectal cancer and related deaths after a colonoscopy with normal findings. JAMA Internal Medicine, 79(2):153-160.
Lieberman, D., et. al. (2016, November 22). Screening for colorectal and evolving issues for physicians and patients: A review. JAMA, 316(20), 2135-2145. Retrieved from Pubmed: https://pubmed.ncbi.nlm.nih.gov/27893135/
Lin, J. S., et. al. (2016). Screening for colorectal cancer: Updated evidence report and systematic review for the US preventative services task force. JAMA, 315(23), 2576-2594. Retrieved from PubMed: https://pubmed.ncbi.nlm.nih.gov/27305422/
Pan, J., et. al. (2016, March). Colonoscopy reduces colorectal cancer incidence and mortality in patients with non-malignant findings: A meta-analysis. American Journal of Gastroenterology, 111(3), 355-365. Retrieved from PMC: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4820666/
Rabeneck, L., & Paszat, L. F. (2010). Circumstances in which colonoscopy misses cancer. Frontline Gastroenterology, 1, 52-58. Retrieved from BMJ: https://fg.bmj.com/content/flgastro/1/1/52.full.pdf
Samadder, N. J. (2014, April).Characteristics of missed or interval colorectal cancer and patient survival: a population-based study. Gastroenterology, 146(4), 950-960. Retrieved from PubMed: https://pubmed.ncbi.nlm.nih.gov/24417818/
Sharma, P., Burke, Burke, C. A., Johnson, D. A., & Cash, B. D. (2020). The importance of colonoscopy bowel preparation for the detection of colorectal lesions and colorectal cancer prevention. Endoscopy International Open, 08(05), E673-E683. Retrieved from Thieme: https://www.thieme-connect.com/products/ejournals/pdf/10.1055/a-1127-3144.pdf