EBM Consult

Lab Test: Fecal Occult (Blood) Level

    Lab Test
    • Fecal Occult Blood Test
    • Detection of occult blood in feces for the evaluation and management of gastrointestinal disorders and colorectal cancers
    Reference Range
    • Adults and children:  Negative
    Indications & Uses
    • Abdominal pain - blood in the stool suggests underlying peptic ulcer disease, polyps, inflammatory bowel disease, vascular ischemia, or intussusception
    • Screening for colorectal cancer - colonoscopy is recommended whenever a patient presents with a positive fecal occult blood test on any specimen. 
    • Suspected gastrointestinal bleeding - a guaiac test for occult blood in stool will detect 0.5 to 1 mg of Hgb per mL of aqueous solution, while the Hematest┬« technique is capable of detecting as little as 0.1 mg of blood in aqueous solution.  The combination of a highly sensitive guaiac test with an immunochemical test may differentiate upper from lower gastrointestinal bleeding.  False results - Betadine┬« or ingestion of preparations containing the expectorant guaifensin will result in a positive guaiac test. Fecal occult blood:  trace.  The Hematest┬« will at times paradoxically give only a trace of 1+ reaction with tarry stools.
    • Suspected infectious diarrhea - blood may be found in any diarrheal stool; however, it is more common in infections caused by pathogens that produce injury to the mucosa, particularly Shingella, Yersinia, enterohemorrhagic E coli, Campylobacter and Salmonella. 
    • Suspected irritable bowel syndrome (IBS) - negative occult blood test results are expected in IBS.  Positive results from a stool specimen may indicate organic or structural disease such as colon cancer requiring diagnostic colonoscopy. 
    • Suspected or known Henoch-Schoniein purpura - fecal occult blood tests may be positive in these patients.
    Clinical Application
    • In normal situations, only small quantities (2 to 2.5 mL) of blood are passed into the GI tractUsually this bleeding is not significant enough to cause a positive result in the stool for occult blood (OB) testing. 
    • Tumors of the intestine grow into the lumen and are subjected to repeated trauma by the fecal stream, which can then cause the friable neovascular tumor to ulcerate and begin bleeding.  Most often, bleeding is so slight that gross blood is not seen in the stool. 
    • Guaiac is the most commonly performed chemical assay.  The peroxidase-like acitivity of hemoglobin catalyzes the reaction of peroxide and a chromogen called orthotolidine to form a blue-stained oxidized orthotolidine. 
    • When testing is properly performed, a positive result obtained on multiple specimens collected on successive days warrants a thorough GI evaluation - usually EGD and colonoscopy. 
    • Positive test results may indicate:
      • GI tumor, peptic diseases (esophagitis, gastritis, and ulceration), varices, inflammatory bowel disease (ulcerative colitis, Crohn's disease), ischemic bowel disease, GI trauma, recent GI surgery, or hemorrhoids and other anorectal problems.
    Related Tests
    • Colonoscopy - this test allows endoscopic evaluation of the entire colon.
    • Esophagogastroduodenoscopy - this endoscopic procedure visualizes the esophagus, stomach, and duodenum.
    • Barium enema - this test uses barium to provide x-ray visualization of the colon.
    • Upper GI series - this test uses barium to provide x-ray visualization of the small intestines.
    • Septin 9 DNA Methylation assay - this blood test is used to screen asymptomatic patients for colorectal cancer.
    Drug-Lab Interactions
    • Vigorous exercise
    • Ingestion of red meat within 3 days before testing.
    • Bleeding gums following a dental procedure or disease may affect results.
    • Ingestion of peroxidase-rich e=vegetables and fruits (turnips, artichokes, mushrooms, radishes, broccoli, bean sprouts, cauliflower, oranges, bananas, cantaloupes, and grapes) and horseradish may affect results. 
    • Drugs that may cause GI bleeding include:  anticoagulants, aspirin, colchicine, iron preparations (large doses), nonsteroidal antiarthritics, and steroids.  Although these drugs do not interfere with the performance of the test, they can cause GI bleeding not associated with pathology. 
    • Drugs that may instigate the peroxidation reaction and cause false-positive results include:  boric acid, bromides, colchicine, iodine, iron, and rauwolfia derivatives.
    • Vitamin C may cause false-negative results by inhibiting the peroxidation reaction.
    • Be gentle in obtaining stool by digital rectal examination.  Traumatic digital examination can cause a false-positive result, especially in patients with prior anorectal disease such as hemorrhoids.
    • Hemoccult Slide Test: 
    • Place stool samples on one side of guaiac paper.  Stool samples should be from two different areas of the specimen.
    • Place two drops of developer on the other side.
    • Note that a bluish discoloration indicates OB in the stool.  
    • Tablet Test:
      • Place a stool sample on the test paper.
      • Place a tablet on top of the stool specimen.
      • Put two or three drops of tap water on the tablet and allow to flow onto the paper.
      • Note that a bluish discoloration indicates OB in the blood. 
    • If the results are positive, inquire whether the patient violated any of the preparation recommendations. 
    • Refer patient for a thorough GI evaluation if results are positive.
    What To Tell Patient Before & After
    • Explain the procedure to the patient.
    • Instruct the patient to refrain from eating any red meat for at least 3 days before the test.
    • Instruct the patient to refrain from drugs known to interfere with OB testing.
    • Instruct the patient in the method of obtaining appropriate stool specimens.  Many procedures are available (e.g., specimen cards, tissue wipes, test paper).  Tests may be done at home with specimen cards (Hemoccult) and mailed to a local testing laboratory or doctor's office when collected. 
    • Instruct the patient not to mix urine with the stool specimen. 
    • Inform the patient about the need for multiple specimens obtained on separate days to increase the test's accuracy.
    • Note that in some centers a high-residue diet is recommended to increase the abrasive effect of the stool. 
    • Inform the patient of the results.
    • LaGow B et al., eds. PDR Lab Advisor. A Comprehensive Point-of-Care Guide for Over 600 Lab Tests.  First ed. Montvale, NJ: Thomson PDR; 2007.
    • Pagana K, Pagana TJ eds. Mosby's Manual of Diagnostic and Laboratory Tests. 5th Ed.  St. Louis, Missouri. 2014.

MESH Terms & Keywords

  • Fecal, Occult