Colorectal Cancer Screening Tests
Colorectal cancer screening is an important part of routine health care. This post is not intended to be a guide for choosing the right test, but rather a guide to help understand and use the medical terms connected with the tests.
Most colorectal cancers begin as a polyp, a growth in the tissue that lines the inner surface of the colon or rectum. A type of polyp known as an adenoma may have a higher risk of becoming cancer.
Two types of tests, stool-based screening tests and visual-based screening exams, can be used to screen for both polyps and colorectal cancer.
Stool-based Screening Tests
- FOBT – fecal (stool) occult bold test
- gFOBT – guaiac fecal occult blood test
- FIT – fecal immunochemical test, also called iFIT
- FIT-DNA – multitarget FIT test, also referred to as Cologuard (a brand of the test approved by the FDA)
Polyps and colorectal cancers can bleed and cause occult (hidden) blood to be present in the stool (feces). Stool-based screening test checks for occult blood. Positive findings may indicate colon cancer or polyps in the colon or rectum — though not all cancers or polyps bleed, and colonoscopy may be recommended for further study.
Stool-based tests, referred to as FOBT (fecal occult bold test), use stool samples that are collected at home and sent to a lab for testing. FOBTs are less invasive, less expensive, and easier to have done than visual exams; however, they need to be done more often.
Two types of FOBT that have been approved by the FDA (Food and Drug Administration), are guaiac FOBT and FIT.
Guaiac FOBT, also called gFOBT, uses the chemical guaiac to check for the presence of occult blood in the stool. However, guaiac may detect blood from foods such as red meat and blood from other organs such as the stomach, resulting in a higher rate of false positives than FIT.
FIT (fecal immunochemical test) uses antibodies to detect blood in the stool. It only detects human blood from the large intestine. Therefore, medicine or food does not interfere with the test. It is more accurate and has fewer false positives than gFOBT.
FIT-DNA, a type of FIT, is often referred to as Cologuard. It is multitargeted and detects occult blood as well as nine DNA biomarkers in three genes that have been found in colorectal cancer and precancerous advanced adenomas in the stool. A computer program analyzes the results of the two tests (blood and DNA biomarkers) and provides a finding of negative or positive. Cologuard is prescription based only and must be ordered by a healthcare provider.
Visual-based Screening Exams
- Colonoscopy – visual exam of the colon
- CT colonography – radiographic imaging of the colon
- Sigmoidoscopy – visual exam of the sigmoid colon and rectum
Visual Exams are used to view the colon and rectum for the presence of polyps and colorectal cancer. Three types of visual exams are used. Exams are done in a medical setting, preparation is required, and sedation may be ordered.
Colonoscopy is a visual exam of the colon (large intestine). In this test, a colonoscope, a long, flexible tube with a tiny camera attached is inserted through the anus into the rectum and the entire colon. Any abnormal growths in the colon and the rectum can be removed during a colonoscopy including growths in the upper parts of the colon that are not reached by sigmoidoscopy. Colonoscopy also is used as a follow-up test if anything unusual is found during one of the other screening tests.
CT (computed tomography) colonography, also called a virtual colonoscopy, uses X-rays and computers to produce images of the entire colon, which are displayed on a computer screen for the doctor to analyze. Polyps or cancers cannot be removed during this procedure.
Sigmoidoscopy is a visual exam of the rectum and sigmoid colon. It is like a colonoscopy except a shorter tube is used to view the lower portion of the large intestines. During a sigmoidoscopy, abnormal growths in the rectum and sigmoid colon can be biopsied. Sigmoidoscopy is not used as commonly as colonoscopy.
For more in-depth learning about the advantages and disadvantages of each of the screening tests click HERE.