Unfortunately, FOBT or fecal occult blood testing does not find those polyps or cancers that are not bleeding. Depending on the sensitivity of the test, it may miss a fair percentage of cancers and most polyps. And the test may be falsely positive because of other conditions that cause intestinal bleeding, requiring unnecessary colonoscopy follow-up.
The latest 2007 Joint Guidelines for Screening recommend fecal occult blood testing as tests that primarily detect cancer.
However, FOBT has been shown in randomized clinical trials to reduce deaths from colorectal cancer by as much as one-third and is an important part of public health screening strategies.
All positive fecal occult blood tests need to be followed-up with a full colonoscopy to look for polyps or cancer.
There are two approaches to FOBT. The older guaiac-based test (gFOBT) measures one part of the hemoglobin molecule. A newer fecal immunochemical test (FIT) measures a different part. Globin, a protein measured by FIT, is only present in when bleeding occurs in the colon or rectum, eliminating false positives from stomach ulcers and bleeding in the the upper digestive tract or meat eaten before the test.
FOBTs are take-home test kits that are completed by patients. It is important that the tests be done accurately, including restricting certain drugs and foods before some tests and taking enough samples. Accuracy improves with the full number of samples and when the test is done every year.
Dietary and drug restrictions: When you use the guaiac-based FOBT such as Hemoccult®, it is important to avoid non-steroidal anti-inflammatory drugs (NSAIDS) such as ibuprofen, naproxen, or more than one aspirin a day for seven days before testing. In addition red meat (beef, lamb, and liver) and vitamin C supplements, citrus fruits, and juices should be avoided for three days prior to the test.
Two small samples of three different bowel movements are smeared onto small paper squares. The kit is then returned to the doctor or mailed to a medical laboratory.
Fecal immunochemical tests (FIT) such as Hemoccult® ICT or InSure® have no drug or dietary restrictions, but it is important to avoid testing during the menstrual period or when there is rectal bleeding or bleeding from the urinary tract.
Samples are collected from bowel movements over two or three consecutive days. Some tests use a stick to collect stool, others use a small brush. None require actually touching the bowel movement.
Digital Rectal Exam
A single test done during a digital rectal exam in a doctor’s office is not sufficient for screening. The United States Preventive Services Task Force says,
Digital Rectal Examination/Office FOBT
There is little evidence to determine the effectiveness of either digital rectal examination or a single office FOBT using a stool sample obtained on DRE. Fewer than 10 percent of colorectal cancers arise within reach of the examining finger, and some of these lesions will already be symptomatic. The sensitivity of a single office FOBT is likely to be substantially lower than that of screening protocols involving multiple test cards: in one study the first test card would have missed 42 percent of cancers detected by screening. Samples collected by DRE may be affected by other limitations, including inadequate amount of stool or trauma from the exam.
Patient Instructions for Some FOBT and FIT screening tests.
Links to instructions are provided for preliminary information for you only. The most current instructions will be included with your test kit. Your doctor may suggest a different brand of test.