Fecal Occult Blood

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

1. See Also
   1. Gastrointestinal Bleeding
   2. Gastrointestinal Occult Bleeding Causes
2. Available Tests
   1. Guaiac-Based Test (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
   2. 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
   3. Immunochemical Fecal Occult Blood: HemeSelect, FECA-EIA
      1. Identifies only intact human Hemoglobin
      2. 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
      3. Disadvantages
         1. Requires increased lab staff time
         2. Detects only lower gastrointestinal tract bleeding
3. Efficacy: Colon Cancer Screening (Guaiac tests)
   1. USPSTF Strength of Recommendation: B
   2. Screening picks up 26% of Colorectal Cancer
   3. Lowers Colon Cancer mortality (versus no screening)
      1. Annual screening: 33% reduction
      2. Biennial (every 2 years): 15-21% reduction
   4. References
      1. Kronberg (1996) Lancet 348:1467-71
      2. Hardcastle (1996) Lancet 348:1472-7
      3. Mandel (2000) N Engl J Med 343:1603-7
      4. Mandel (1999) J Natl Cancer Inst 91:434-7
4. Indications
   1. Start Occult blood screening by age 50 years old (or per Colorectal Cancer Screening protocol)
5. 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
6. 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
7. Causes: Positive occult blood test
   1. Gastrointestinal Occult Bleeding Causes
8. References
   1. Rockey in Feldman (2002) Sleisenger GI, p. 232-48
   2. Herold in Rakel (2002) Family Practice, p. 1563-4