F
Fecal DNA assay
Basics the nurse needs to know
Cancer of the colon is believed to develop over a period of many years, as a benign polyp mutates and becomes a malignant tumor. During this time period, several genetic mutations develop, affecting both oncogene and tumor suppressor genes. As rapidly growing epithelial cells shed continuously into the fecal matter, the cells contain the DNA mutations. The cells are degraded, but the altered DNA markers remain intact in the feces (Sanford & McPherson, 2009).
Precancerous polyps only bleed intermittently, making it difficult to detect their presence by fecal occult blood testing. The fecal DNA assay test has a much higher sensitivity than the fecal occult blood test because of the continuous presence of DNA mutations in the feces. However, of all who have a positive fecal DNA test result, only 2% were shown to have colorectal cancer when the follow-up colonoscopy was performed (Sanford & McPherson, 2009).
Currently, the high cost of the test and less insurance coverage for this method of testing make it difficult to use this test for mass screening for all people. In addition, only reference laboratories are equipped to analyze the specimens. There are no current recommendations as to how often a person should be screened by this method of testing (Mahon, 2009).
How the test is done
A special test collection kit must be used. The patient defecates into a large special container that fits over the toilet seat. The container with the feces is placed inside a larger container with a lid. The container is mailed to the reference laboratory, using the required cooler with its freezer packet inside.
Fecal elastase-1
Also called: Pancreatic Elastase-1
Fecal occult blood tests
Basics the nurse needs to know
Occult blood refers to blood that is present in feces, but is not visible. In some cases, the blood is not seen because the amount is small and mixed in the feces. Also, precancerous polyps bleed intermittently and blood is not always present in a single specimen. There are two basic test methods to screen for detection of fecal occult blood. These are: the older Guaiac method (Hemoccult Sensa™) and the newer fecal immunochemical test method (FIT). Both methods provide for collection of the fecal samples at home. The specimens can then be mailed or taken to the physician’s office or laboratory for analysis. Each of these FOBT methods has been approved as a screening test for colorectal cancer and when this method of screening is used, the test should be done annually (Mahon, 2009). If the FOBT result is positive, a colonoscopy is recommended.
Guaiac method
This method has many false-positive results. A false positive means that the test showed a positive value, but the patient did not have occult blood in the feces. Eating red meat will cause a positive result because the red meat and blood contain animal hemoglobin and heme molecules. Some fruits and vegetables contain a vegetative source of peroxidase and will cause a false-positive result. Some medications and alcohol can cause slight bleeding in the intestinal mucosa and bleeding from that source cannot be distinguished from bleeding from a polyp or tumor (Box 6). Vitamin C, as a supplement or in citrus fruits, can cause a false-negative test result. A false negative means that the test result is negative, but the patient actually has blood in the fecal sample.
Immunochemical method
This method is based on antihuman hemoglobin antibodies that react with the undegraded globulin molecules of hemoglobin in the feces. In the test, immunochemicals are placed in contact with the fecal smears. If hemoglobin is present in the feces, there will be an antibody response and a pink color test line will appear on the testing strip. This test method only detects hemoglobin that originated from a bleeding site in the colon. It cannot detect hemoglobin that came from bleeding sites in the upper gastrointestinal tract or small intestine. It does not react to peroxidase from fruits and vegetables or from hemoglobin that comes from eating red meat. The FIT test has many fewer false-positive results than tests using the guaiac method (Sanford & McPherson, 2009).
Interfering factors
Guaiac method and immunochemical method
NURSING CARE
Guaiac method only: Instruct the patient to avoid the foods presented in Box 6 for 3 days before the test. Vitamin C must be omitted for 5 days before the test. If possible, the medications that cause intestinal irritation should be stopped for 7 days before the test. To avoid intestinal irritation, alcohol should also be omitted.
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