Fecal (stool) tests are used to detect colon cancer. Only very small stool samples are needed for these non-invasive cancer tests. There are several different methods currently in use.
- Fecal Occult Blood Tests (FOBT)
- Fecal DNA Test
Fecal occult blood tests (FOBT) are able to detect small hidden (occult) blood present in stools, a possible sign of colon cancer or colonic polyps (precursors of cancer). Occult blood refers to very small amounts of blood that cannot be seen by visual inspection, but can be detected using chemical tests. The detection of blood in stool does not necessarily indicate cancer, it simply helps health care professionals make an accurate diagnosis. There are currently two methods used to detect occult blood, they are described below.
The GFOBT detects the activity of peroxidase, an enzyme found in the blood. The test can be done using a home kit obtained from a health care provider. It requires a sample taken of the inside and the outside of three separate bowel movements. The samples are then placed on appropriate slides and returned for testing. Some foods, medications, and supplements can cause false positives, so be sure to carefully follow instructions from your health care provider or the test manufacturer.1 2
The fecal immunochemical test is a more advanced, easier to use FOBT that has shown a better combination of sensitivity and specificity. The test uses antibodies to detect the presence of blood hemoglobin. This method is not affected by food, medication, or supplements and it is also easier for subjects to obtain an adequate sample. The FIT only requires samples from the surface of two bowel movements. Although the FIT is more user friendly, it is still important to carefully follow your health care providers instructions.1 2
As part of the normal turnover that occurs in the cells that line the bowel, many of the epithelial cells that line the intestines are shed into the waste. As discussed in the section on tumor suppressors, many colon cancers share a common genetic defect in the APC gene. The cells that are shed into the fecal matter contain DNA and tests have been developed to detect mutations in the APC gene. This technique is able to detect the DNA from colon cancer cells even at early stages of cancer. Its non-invasive nature makes it a promising new screening tool.3 4
In 2014, the FDA approved the first stool DNA test for colon cancer, Cologuard®.5 6 7 The test detects mutations in genes associated with colorectal cancer, and modifications (methylation) of other cancer genes. The test also detects the presence of hemoglobin (blood) in stool. Like any medical test, Cologuard® is not 100% accurate. In a large clinical trial, the test was able to detect 92% of stage 1-4 colorectal cancers.8 Because the genetic changes seen in cancer happen over time, the test can also detect pre-cancerous conditions. In the clinical trial, the test was able to detect 69% of 'high risk' pre-cancers, those thought to be most likely to progress to cancer.8
- 1 a b Guittet L et al. Comparison of a guaiac based and an immunochemical faecal occult blood test in screening for colorectal cancer in a general average risk population. Gut 2007 Feb; 56:210-4. [PUBMED]
- 2 a b A Smith, GP Young, SR Cole, P Bampton. Comparison of a Brush-Sampling Fecal Immunochemical Test for Hemoglobin With a Sensitive Guaiac-Based Fecal Occult Blood Test in Detection of Colorectal Neoplasia. Cancer. 2006;107:21529. [PUBMED]
- 3Calistri D, Rengucci C, Casadei Gardini A, Frassineti GL, Scarpi E, Zoli W, Falcini F, Silvestrini R, Amadori D. Fecal DNA for noninvasive diagnosis of colorectal cancer in immunochemical fecal occult blood test-positive individuals. Cancer Epidemiol Biomarkers Prev. 2010 Oct;19(10):2647-54. [PUBMED]
- 4Ahlquist DA. Molecular detection of colorectal neoplasia. Gastroenterology. 2010 Jun;138(6):2127-39. [PUBMED]
- 5FDA approval of Cologuard. http://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm409021.htm Accessed 10-20-2016
- 6FDA Cologuard fact sheet http://www.fda.gov/MedicalDevices/ProductsandMedicalProcedures/DeviceApprovalsandClearances/Recently-ApprovedDevices/ucm410569.htm Accessed 10-20-2016
- 7Cologuard manufacturer's site http://www.cologuardtest.com/ Accessed 10-20-2016
- 8 a b Imperiale TF, Ransohoff DF, Itzkowitz SH, Levin TR, Lavin P, Lidgard GP, Ahlquist DA, Berger BM. Multitarget stool DNA testing for colorectal-cancer screening. N Engl J Med. 2014 Apr 3;370(14):1287-97. doi: 10.1056/NEJMoa1311194. Epub 2014 Mar 19.