fecal occult blood test


Fecal Occult Blood Test

 

Definition

The fecal occult blood test (FOBT) is performed as part of the routine physical examination during the examination of the rectum. It is used to detect microscopic blood in the stool and is a screening tool for colorectal cancer.

Purpose

FOBT uses chemical indicators on stool samples to detect the presence of blood not otherwise visible. (The word "occult" in the test's name means that the blood is hidden from view.) Blood originating from or passing through the gastrointestinal tract can signal many conditions requiring further diagnostic procedures and, possibly, medical treatment. These conditions may be benign or malignant and some of them include:
  • colon cancer, rectal cancer, and gastric cancers
  • ulcers
  • hemorrhoids
  • polyps
  • inflammatory bowel disease
  • irritations or lesions of the gastrointestinal tract caused by medications (such as nonsteroidal anti-inflammatory drugs, also called NSAIDs)
  • irritations or lesions of the gastrointestinal tract caused by stomach acid disorders, such as reflux esophagitis
The FOBT is used routinely (in conjunction with a rectal examination performed by a physician) to screen for colorectal cancer, particularly after age 50. The ordering of this test should not be taken as an indication that cancer is suspected. The FOBT must be combined with regular screening endoscopy (such as a sigmoidoscopy) to detect cancers at an early stage.

Precautions

Certain foods and medicines can influence the test results. Some fruits contain chemicals that prevent the guaiac, the chemical in which the test paper is soaked, from reacting with the blood. Aspirin and some NSAIDs irritate the stomach, resulting in bleeding, and should be avoided prior to the examination. Red meat and many vegetables and fruits containing vitamin C also should be avoided for a specified period of time before the test. All of these factors could result in a false-positive result.

Description

Feces for the stool samples is obtained either by the physician at the rectal examination or by the patient at home, using a small spatula or a collection device. In most cases, the collection of stool samples can easily be done at home, using a kit supplied by the physician. The standard kit contains a specially prepared card on which a small sample of stool will be spread, using a stick provided in the kit. The sample is placed in a special envelope and either mailed or brought in for analysis. When the physician applies hydrogen peroxide to the back of the sample, the paper will turn blue if an abnormal amount of blood is present.

Types of fecal occult blood tests

Hemoccult is the most commonly used fecal occult blood test. The Hemoccult test takes less than five minutes to perform and may be performed in the physician's office or in the laboratory. The Hemoccult blood test can detect bleeding from the colon as low as 0.5 mg per day.Tests that use anti-hemoglobin antibodies (or immunochemical tests) to detect blood in the stool are also used. Immunochemical tests can detect up to 0.7 mg of hemoglobin in the stool and do not require dietary restrictions. Immunochemical tests
  • are not accurate for screening for stomach cancer
  • are more sensitive than Hemoccult tests in detecting colorectal cancer
  • are more expensive than Hemoccult tests.
Hemoquant, another fecal occult blood test, is used to detect as much as 500 mg/g of blood in the stool. Like the Hemoccult, the Hemoquant test is affected by red meat. It is not affected by chemicals in vegetables.Fecal blood may also be measured by the amount of chromium in the red blood cells in the feces. The stool is collected for three to ten days. The test is used in cases where the exact amount of blood loss required. It is the only test that can exclude blood loss from the gastrointestinal area with accuracy.Medicare coverage began on January 1, 2004, for a newer fecal occult blood test based on immunoassay. This technique does not rely on guiaic, so it is not influenced by diet or medications used prior to the test. The immunoassay test also requires fewer specimen collections. At a conference of gastroenterologists (physicians who specialize in diseases of the stomach and related digestive systems), a company announced a new fecal occult blood test that was based on DNA and appeared more sensitive than traditional tests. Widespread use of these new tests remains to be seen; the traditional guiaic test has been in place for about 30 years.

Preparation

For 72 hours prior to collecting samples, patients should avoid red meats, NSAIDs (including aspirin), antacids, steroids, iron supplements, and vitamin C, including citrus fruits and other foods containing large amounts of vitamin C. Foods like uncooked broccoli, uncooked turnips, cauliflower, uncooked cantaloupe, uncooked radish and horseradish and parsnips should be avoided and not eaten during the 72 hours prior to the examination. Fish, chicken, pork, fruits (other than melons) and many cooked vegetables are permitted in the diet.

Results

Many factors can result in false-positive and falsenegative findings.

Positive results

It is important to note that a true-positive finding only signifies the presence of blood—it is not an indication of cancer. The National Cancer Institute states that, in its experience, less than 10% of all positive results were caused by cancer. The FOBT is positive in 1-5% of the unscreened population and 2-10% of those are found to have cancer. The physician will want to follow up on a positive result with further tests, as indicated by other factors in the patient's history or condition.

Negative results

Alternatively, a negative result (meaning no blood was detected) does not guarantee the absence of colon cancer, which may bleed only occasionally or not at all. (Only 50% of colon cancers are FOBT-positive.)

Conclusions

Screening using the FOBT has been demonstrated to reduce colorectal cancer. However, because only half of colorectal cancers are FOBT-positive, FOBT must be combined with regular screening endoscopy to increase the detection of pre-malignant colorectal polyps and cancers. Since, through FOBT, cancer may be detected early, the benefits of possible early detection must be considered along with the likelihood of complications and costs for additional studies.

Resources

Books

DeVita, Vincent, Samuel Hellman, and Steven Rosenburg. Cancer: Principles and Practices of Oncology. Philadelphia: Lippincott Williams & Wilkins, 2001.Yamada, Tadetaka, editor. Textbook of Gastroenterology Volumes One and Two. Philadelphia: Lippincott Williams & Wilkins, 2001.

Periodicals

"DNA-based Stool Test More Sensitive than Fecal Occult Blood Test in Study." Health & Medicine Week November 10, 2003: 194.From the Centers for Disease Control and Prevention. "Trends in Screening for Colorectal Cancer—United States, 1997 and 1999." Journal of the American Medical Association 28 (March 2001): 12.Silverman, Jennifer. "Colorectal Screning Option." Family Practice News 33 (December 15, 2003): 49-51.

Organizations

American Cancer Society. 1599 Clifton Road NE, Atlanta, GA 30329. (800)ACS-2345. http://www.cancer.org.National Cancer Institute (National Institutes of Health). 9000 Rockville Pike, Bethesda, MD 20892. (800) 422-6237. http://www.nci.nih.gov.

Other

"Colon and Rectum Cancer." American Cancer Society. 2000. July 10, 2001. http://www.cancer.org."Colorectal Cancer Screening." WebMD. 〈http://my.webmd.com/content/article/2955.291〉."Fecal Occult Bood Test." Virtual Health Fair. July 10, 2001. 〈http://vfair.com/resources/lab/fecal.htm〉.

Key terms

Occult — Not visible or easily detected.

fecal occult blood test

(fē′kăl) A screening test for disorders of the gastrointestinal tract, including anemias that may be caused by gastrointestinal blood loss, e.g., cancer of the colon. Traditionally, a small amount of feces is collected on cards accompanying several consecutive bowel movements. These cards are submitted to a lab for analysis. Alternatively, a flushable pad is dropped into the toilet bowl after a bowel movement. A change in color (to blue or green) in the pad reveals occult blood if present.

Patient care

Patients should be taught how to collect and protect card specimens submitted to the laboratory. They should avoid taking nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen or aspirin and should not take more than one adult aspirin (325 mg) per day for several days before and during specimen collection. They should also avoid red meats (including cold cuts) for 3 days before and during the testing period (since the test detects animal as well as human blood), and limit vitamin C intake to less than 250 mg/day. The patient should avoid contaminating the stool sample by not allowing it to touch the toilet or the water. The bowel movement should be made into a clean, dry container or onto the flushable collection tissue supplied with the kit. A wooden stick or swab transfers a small quantity of the specimen to the collection card. Specimens should be protected from heat, light, and chemicals. When developing the test, the manufacturer's instructions should be followed precisely. Gloves should be worn and standard precautions followed. If the specimen is fresh, 3 to 5 min should elapse before adding developer to make sure the stool has penetrated the paper. For all guaiac tests, a blue color indicates the presence of blood. Blue-green also may indicate blood, while green alone is considered negative. After reading the results, the positive and negative controls should be tested to verify test accuracy. The results are then documented. Care should be taken to be sure that a test kit is not used after its expiration date. Specimens should not be collected or the test run if blood can be seen in the stool or urine, if the patient is menstruating, or if a urinary tract infection is present.

CAUTION!

The developer bottle should be stored in a lab area and not in a patient area because it could be mistaken for a liquid medication or a toy.