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Fecal Occult Blood

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Fecal Occult Blood, Fecal Occult Blood Test, Stool Guaiac, Stool Occult Blood, FOBT, Hemoccult, Heme-Porphyrin Test, Hemoquant, Immunochemical Fecal Occult Blood, HemeSelect, FECA-EIA

  • Indications
  1. Acute Gastrointestinal Bleeding
  2. Colon Cancer screening
    1. Start Occult blood screening by age 50 years old (or per Colorectal Cancer Screening protocol)
  • Preparations
  1. Guaiac-Based Test (gFOBT, Hemoccult)
    1. Reagent derived from wood resin of Guajacum trees
    2. Reaction based on presence of Hemoglobin in sample
      1. Hemoglobin contains pseudoperoxidase
      2. Pseudoperoxidase converts guaiac to blue
      3. Reaction requires addition of hydrogen peroxide
    3. Most commonly used test (for both GI Bleeding, Colorectal Cancer Screening)
      1. Do not use office based testing for screening
    4. Efficacy for gFOBT in Colorectal Cancer Screening (based on 3 home samples submitted)
      1. Test Sensitivity: 62-79%
      2. Test Specificity: 87-96%
      3. Positive Likelihood Ratio: 4.7 to 19.8
      4. Negative Likelihood Ratio: 0.2 to 0.4
      5. Holme (2013) Cochrane Database Syst Rev (9):CD009259 [PubMed]
  2. Fecal Immunochemical Test (FIT, HemeSelect, FECA-EIA, OC-Micro, OC-Sensor, OC-Light)
    1. Identifies only intact human Hemoglobin
    2. Efficacy in screening for Colorectal Cancer
      1. Test Sensitivity: 73-89%
      2. Test Specificity: 92-95%
      3. Lee (2014) Ann Intern Med 160(3): 171-81 [PubMed]
    3. Advantages
      1. Ingested red meat does not give false positive
      2. Food-based peroxidases do not give false positives
      3. Specific for lower gastrointestinal tract bleeding
    4. Disadvantages
      1. Requires increased lab staff time
      2. Detects only lower gastrointestinal tract bleeding
  3. Fecal Immunochemical with DNA Test (FIT-DNA, ColoGuard)
    1. Combination of FIT testing (see above) and DNA biomarkers
    2. Much more expensive than FIT Testing alone
    3. Efficacy in screening for Colorectal Cancer
      1. Test Sensitivity: 92%
      2. Test Specificity: 90%
      3. Imperiale (2014) N Engl J Med 370(14): 1287-97 [PubMed]
  4. Heme-Porphyrin Test (Hemoquant)
    1. Quantifies porphyrins from Hemoglobin
    2. Advantages
      1. Measures amount of stool blood present
      2. Food-based peroxidases do not give false positives
    3. Disadvantages
      1. Expensive and requires increased lab staff time
      2. High False Positive Rate
  • Technique
  1. Use high-sensitivity guaiac-based tests (Hemoccult Sensa)
  2. Minimize False Positive Rate
    1. See False positives below
    2. Avoid listed items for 3 days prior to testing
    3. Aspirin and NSAIDs appear to be safe prior to FOBT
      1. (2005) Prescriber's Letter p. 15
  3. Minimize False Negative Rate
    1. Avoid Vitamin C >250 mg/day for 2 days before test
    2. Test 2 samples from each of 3 consecutive stools
    3. Space samples over 4-6 days
  4. Digital Rectal Exam does not increase false positives
    1. Bini (1999) Arch Intern Med 159:2022-6 [PubMed]
  • Causes
  • False positive Guaiac tests (Hemoccult)
  1. Foods associated with false positive FOBT
    1. Common Causes
      1. Red meat
      2. Cantaloupe and other melon
      3. Grapefruit
      4. Figs
      5. Broccoli
      6. Turnips
      7. Radishes
      8. Horseradish
      9. Cauliflower
    2. Uncommon causes of false positive FOBT
      1. Cucumber
      2. Carrot
      3. Cabbage
      4. Potato
      5. Pumpkin
      6. Zucchini
      7. Parsley
  2. Medications and Supplements
    1. Previously ASA NSAIDS thought to cause false positive
      1. Now both are thought to be safe prior to FOBT
      2. Avoid doses of ASA >1250 mg/day
    2. Medications that do not result in false positives
      1. Coumadin
  3. Miscellaneous causes of false positive FOBT
    1. Nasal bleeding source (e.g. Epistaxis)
    2. Oral Bleeding source
      1. Gingival Bleeding
      2. Tonsillitis
    3. Pulmonary bleeding source (e.g. Hemoptysis)
    4. Long distance Running
  • Causes
  • Positive occult blood test
  • References
  1. Rockey in Feldman (2002) Sleisenger GI, p. 232-48
  2. Herold in Rakel (2002) Family Practice, p. 1563-4
  3. Wilkins (2018) Am Fam Physician 97(10): 658-65 [PubMed]