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FECAL OCCULT BLOOD TESTING (FOBT) is the most frequently performed fecal analysis. Screening detects occult, or hidden, blood in the stool. Bleeding in the upper gastrointestinal (GI) tract may produce black, tarry stool; bleeding in the lower GI tract may result in overtly bloody stool (see Common sites and causes of GI blood loss). However, no visible signs of bleeding may be present at the early stages of carcinoma development or GI diseases. For this reason, FOBT can help with early identification of carcinomas and diseases of the GI tract.

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Let's take a look at what you need to know about this important diagnostic test.


Preparation station

Here's what you need to do before collecting the stool sample:


[black small square] Explain to your patient the purpose of the procedure.


[black small square] If your patient is a woman, testing shouldn't be done during or up to 3 days after her menstrual period.


[black small square] Recommend that your patient consume a high-fiber diet, starting 48 to 72 hours before and continuing throughout the collection period. His diet may include the following: small amounts of chicken, turkey, and tuna; generous amounts of both raw and cooked vegetables; plenty of fruits, especially prunes; bran and bran-containing cereals; and moderate amounts of peanuts and popcorn daily. He must avoid red meats, vegetables and fruits with high peroxidase activity (such as turnips, horseradish, and cantaloupe), and large amounts of vitamin C for 48 to 72 hours before specimen collection. Aspirin and nonsteroidal anti-inflammatory drugs may cause a false-positive result.


[black small square] He shouldn't receive a barium enema for 72 hours before or during the collection period.



Collection day

Because there's a risk for false-positive or false-negative results, patient preparation and specimen collection must be followed explicitly (see Factors that can interfere with FOBT). To collect the sample:


* Obtain a random stool specimen while observing standard precautions.


* Follow the manufacturer's instructions for use on how to obtain the correct specimen and perform the test. Two types of FOBT are the traditional guaiac smear (Hemoccult, Seracult, or ColoScreen) and flushable reagent pads (EZ Detect or Colocare).



Results at the ready

An average, healthy person has 2 to 2.5 mL of blood in his stool each day; more than this amount of blood in the stool in a 24-hour period is significant.


Positive testing for occult blood may be caused by these conditions:


[black small square] carcinoma of the colon, rectum, or stomach (Keep in mind that a negative test doesn't completely rule out colorectal disease. Sigmoidoscopy, colonoscopy, or a double contrast barium enema is recommended in addition to FOBT to properly diagnose colorectal disease.)


[black small square] ulcerative colitis and other inflammatory lesions


[black small square] adenomas


[black small square] diaphragmatic hernia


[black small square] peptic ulcer


[black small square] gastritis


[black small square] vasculitis


[black small square] amyloidosis


[black small square] hemorrhoids.



After testing is complete, tell your patient that he may resume a normal diet. Interpret the test results and record your findings. Advise him that further testing and follow-up may be needed.


Early bird special

FOBT is a diagnostic test that's helpful in the early detection of carcinomas and diseases of the GI tract. And with early detection comes prompt treatment!!


Common sites and causes of GI blood loss

Illustrated here are the potential causes of gastrointestinal (GI) blood loss, resulting in a positive fecal occult blood test. Further clinical assessment and testing are necessary to determine the area involved.


Factors that can interfere with FOBT

False-positive results

Drugs and substances that may cause a false-positive fecal occult blood test (FOBT) result include:


* aspirin


* steroids


* indomethacin


* nonsteroidal anti-inflammatory drugs


* boric acid


* bromides


* colchicine


* iodine or povidone-iodine.



Foods that may cause a false-positive FOBT result include:


* red meats, including processed meats and liver


* vegetables and fruits with peroxidase activity, such as turnips, horseradish, mushrooms, broccoli, and radishes and apples, bananas, and cantaloupe.



False-negative results

Substances that may cause a false-negative FOBT result include:


* ascorbic acid (vitamin C) in excess of 250 mg/day


* iron supplements that contain vitamin C in excess of 250 mg.



Other factors affecting test results


* Bleeding hemorrhoids


* Specimen collection during menstrual period


* Hematuria (blood in urine)


* Toilet bowl cleansers


Learn more about it


Fischbach F. A Manual of Laboratory and Diagnostic Tests, 7th edition. Philadelphia, Pa., Lippincott Williams & Wilkins, 2004:274-277.


MedlinePlus. Fecal occult blood test (FOBT). Accessed February 26, 2008.


Nursing: Deciphering Diagnostic Tests. Philadelphia, Pa., Lippincott Williams & Wilkins, 2007:492-495.


Smeltzer SC, et al. Brunner and Suddarth's Textbook of Medical-Surgical Nursing, 11th edition. Philadelphia, Pa., Lippincott Williams & Wilkins, 2007:1130.