Fecal occult blood


Fecal occult blood refers to blood in the feces that is not visibly apparent. A fecal occult blood test checks for hidden blood in the stool. It is a screening test whose only purpose is for colorectal cancer.
Other tests look for globin, DNA, or other blood factors including transferrin, while conventional stool guaiac tests look for heme.

Medical uses

Fecal occult blood testing, as its name implies, aims to detect subtle blood loss in the gastrointestinal tract, anywhere from the mouth to the colon. Positive tests may result from either upper gastrointestinal bleeding or lower gastrointestinal bleeding and warrant further investigation for peptic ulcers or a malignancy. The test does not directly detect colon cancer but is often used in clinical screening for that disease, but it can also be used to look for active occult blood loss in anemia or when there are gastrointestinal symptoms.
The stool guaiac test for hidden blood in the stool should be used at home following the test kits directions with spontaneously passed stool or on samples submitted to a clinical laboratory. Testing kits are available at pharmacies in some countries without a prescription, or a health professional may order a testing kit for use at home. If a home fecal occult blood test detects blood in the stool it is recommended to see a health professional to arrange further testing.

Colorectal cancer screening

An estimated 1–5% of large tested populations have a positive fecal occult blood test. Of those, about 2–10% have cancer, while 20–30% have adenomas. Screening methods for colon cancer depend on detecting either precancerous changes such as certain kinds of polyps or on finding early and thus more treatable cancer. The extent to which screening procedures reduce the risk of gastrointestinal cancer or deaths depends on the rate of precancerous and cancerous disease in that population. gFOBT and flexible sigmoidoscopy screening have each shown benefit. Other colon cancer screening tools such as iFOBT or colonoscopy are also included in guidelines.
In 2009 the American College of Gastroenterology suggested that colon cancer screening modalities that are also directly preventive by removing precursor lesions should be given precedence, and prefer a colonoscopy every ten years in average-risk individuals, beginning at age 50. The ACG suggests that cancer detection tests such as any type of FOB are an alternative that is less preferred, and if a colonoscopy is declined, the FIT should be offered instead. The 2017 US Multi-Society Task Force 's recommended first-tier tests are a colonoscopy every 10 years or annual FIT test. If FIT is utilized, proper steps must be taken to ensure appropriate use and follow up of abnormal FIT results. FIT tests however are not that useful in picking up adenomas, even when advanced.
The United States Preventive Services Task Force 's 2016 recommendation, instead of emphasizing specific screening approaches, has instead chosen to highlight that there is convincing evidence that colorectal cancer screening substantially reduces deaths from the disease among adults aged 50 to 75 years and that not enough adults are using this effective preventive intervention. The ACG and MSTF also included CT colonography every five years, and fecal DNA testing as considerations. All three recommendation panels recommended replacing any older low-sensitivity, guaiac-based fecal occult blood testing with either newer high-sensitivity guaiac-based fecal occult blood testing or fecal immunochemical testing. MSTF looked at six studies that compared high sensitivity gFOBT to FIT, and concluded that there was no clear difference in overall performance between these methods.
The American College of Gastroenterology has recommended the abandoning of gFOBT testing as a colorectal cancer screening tool, in favor of the fecal immunochemical test. Though the FIT test is preferred, even the guaiac FOB testing of average risk populations may have been sufficient to reduce the mortality associated with colon cancer by about 25%. With this lower efficacy, it was not always cost effective to screen a large population with gFOBT.
If colon cancer is suspected in an individual fecal occult blood tests may not be clinically helpful. If a doctor suspects colon cancer, more rigorous investigation is necessary, whether or not the test is positive.
In 2006, the Australian Government introduced the National Bowel Cancer Program which has been updated several times since; targeted screening will be done of all Australians aged from 50 to 74 by 2020. Cancer Council Australia recommended that FOBT should be done every two years. People over 50 not yet eligible for the national program can arrange with their doctor for a FOBT.
The Canadian Cancer Society recommends that men and women aged 50 and over have a FOBT at least every two years.
In colon cancer screening, using only one sample of feces collected by a doctor performing a digital rectal examination is discouraged.
The use of the M2-PK Test is encouraged over gFOBT for routine screening as it may pick up tumors whether or not they are that are bleeding. It is able to pick up 80 percent of colorectal cancer and 44 percent for adenoma > 1 centimeter, while gFOBT picks up 13 to 50 percent of colorectal cancers.

Testing secretions for blood

The use of the FOBT for bleeding from the mouth, nose, esophagus, lungs, stomach and the initial portion of the small intestine, while the same as fecal testing, is discouraged, due to technical considerations including poorly characterized test performance characteristics such as sensitivity, specificity, and analytical interference. However, chemical confirmation that coloration is due to blood rather than coffee, beets, medications, or food additives can be of significant clinical assistance.

Marathon runners

Gastrointestinal complaints and low intensity GI bleeding frequently occur in marathon runners. Strenuous exercise, particularly in elite athlete runners and less frequently in other exercise activities, can cause acute incapacitating gastrointestinal symptoms including heartburn, nausea, vomiting, abdominal pain, diarrhea and gastrointestinal bleeding. Approximately one third of endurance runners experience transient but exercise limiting symptoms, and repetitive gastrointestinal bleeding occasionally causes iron deficiency and anaemia. Runners can sometimes experience significant symptoms including hematemesis. Exercise is associated with extensive changes in gastrointestinal tract physiology, including diversion of blood flow from the GI tract to muscle and lungs, decreased GI absorption and small intestinal motility, increased colonic transit, neuroimmunoendocrine changes in hormones and peptides such as vasoactive intestinal peptide, secretin and peptide-histidine-methionine. Substantial changes occur in stress hormones including cortisol, in circulating concentrations and metabolic behavior of various leucocytes, and in immunoglobulin levels and major histocompatibility complex expression. Symptoms can be exacerbated by dehydration or by pre-exercise ingestion of certain foods and hypertonic liquids, and lessened by adequate training.
Ingestion of 800mg of cimetidine two hours before running a marathon did not significantly affect the frequency of gastrointestinal symptoms or occult gastrointestinal bleeding. Conversely, 800mg of cimetidine 1 hr before the start and again at 50 miles of a 100-mile running race substantially decreased GI symptoms and post-race guaiac test positivity but did not affect race performance.

Methodology

There are four methods in clinical use for testing for occult blood in feces. These look at different properties, such as antibodies, heme, globin, or porphyrins in blood, or at DNA from cellular material such as from lesions of the intestinal mucosa.
FIT testing has replaced most gFOBT tests as the colon cancer screening test of choice. This methodology can be adapted for automated test reading and to report quantitative results, which are potential factors in design of a widescale screening strategy. The number of fecal samples submitted for FIT may affect the clinical sensitivity and specificity of the methodology. High sensitivity gFOBT tests such as Hemoccult SENSA remains an accepted option; and may retain a role in monitoring gastrointestinal conditions such as ulcerative colitis; however the FIT test is preferred in recent guidelines.
Additional methods of looking for occult blood are being explored, including transferrin dipstick and stool cytology.

Test performance

Gastrointestinal blood loss

In healthy people about 0.5 to 1.5ml of blood escapes blood vessels into the stool each day. Significant amounts of blood can be lost without producing visible blood in the stool, estimated as 200ml in the stomach, 100ml in the duodenum, and lesser amounts in the lower intestine. Tests for occult blood identify lesser blood loss.

Clinical sensitivity and specificity

Fecal immunochemical testing can identify as little as 0.3ml of daily blood in the stool; yet this test threshold doesn't cause undue false positives from normal upper intestinal blood leakage because it does not detect occult blood from the stomach and upper small intestine. Thus the FIT test is much more specific for bleeding from the colon or lower gastrointestinal tract than alternatives. The detection rate of the test decreases if the time from sample collection to laboratory processing is delayed; processing the sample in under five days from collection is recommended. It does not appear to be affected by aspirin, anticoagulants, or a nonsteroidal anti-inflammatory drug.
Stool guaiac test for fecal occult blood sensitivity varies depending on the site of bleeding. Moderately sensitive gFOBT can pick up a daily blood loss of about 10ml, and higher sensitivity gFOBT can pick up lesser amounts, requires at least 2ml. to become positive. The sensitivity of a single stool guaiac test to pick up bleeding has been quoted at 10 to 30%, but if a standard three tests are done as recommended the sensitivity rises to 92%. Reduced patient compliance with the collection of three samples hampers the usefulness of this test. Further discussion of sensitivity and specificity issues that relate particularly to the guaiac method is found in the stool guaiac test article.
Fecal porphyrin quantification by HemoQuant can yield a false positive result due to exogenous blood and various porphyrins. HemoQuant is the most sensitive test for upper gastrointestinal bleeding and therefore may be most appropriate fecal occult blood test to use in the evaluation of iron deficiency Advised to stop red meat and aspirin for three days prior to specimen collection False positives can occur with myoglobin, catalase, or protohemes and in certain types of porphyria.
Fecal DNA tests as of 2008 had not been well studied enough to support widespread use.

Regulation

Safety regulations from US accreditor the Joint Commission may have unintentionally decreased digital rectal examination and FOBT in hospital settings such as Emergency Departments.