Faecal occult blood testing (summary section)

• faecal occult blood test (FOBT) is a well-established community-based screening tool for the early detection of colorectal cancer
  
  
• Faecal occult blood testing is of no benefit in the investigation of iron deficiency anaemia (1)
  
  
• two types of test have been used, a guaiac (chemical) test that detects the pseudoperoxidase activity of haemoglobin, and an immunological test using antibodies against human haemoglobin
  • the immunological test
    • more sensitive and specific test for colorectal cancer than the guaiac test, because the former relies upon antibody detection of the globin component of human haemoglobin. Globin is rapidly degraded by gastric acid and digestive enzymes, ensuring that a positive result should only indicate lower gastrointestinal tract bleeding
    • the immunological test is more specific than the guaiac test because ingested animal and plant peroxidases can also survive intestinal transit leading to potentially false positive results and ingested vitamin C can cause false negative results
  • in contrast, the guaiac test detects haem which is far less susceptible to degradation during gastrointestinal transit
    • if upper gastrointestinal bleeding occurs, the guaiac test can return a false positive result for colorectal cancer
  • advice regarding food and medication prior to the faecal occult blood test (2)
    • in order to maximize the accuracy of the test, the guaiac test requires dietary restriction for 72 h before the test to prevent false positive results, namely abstinence from:
  • red meat
  • melons
  • raw turnips
  • radishes
  • broccoli
  • cauliflower
  • also should be avoidance of vitamin C tablets because of higher rates of falsely negative tests
  • for both tests, patients are required to be free from any medications that may cause gastrointestinal bleeding, including:
    • antiplatelets
    • anticoagulant therapy
    • anti-inflammatories
    • selective serotonin reuptake inhibitors

A middle-aged person who has a positive result on an initial faecal occult blood test without slide rehydration has about a 7% to 14% probability of having early-stage (Dukes stage A or B) colorectal cancer (3)

Sensitivity of fecal occult blood tests for important neoplasms is limited (probably between 30% and 50%); therefore, a negative result cannot rule out colorectal cancer (3)

Sensitivity for a single guaiac faecal occult blood test is reduced by about two thirds compared to use of the three guaiac faecal occult blood samples:

• audit data revealed that sensitivity for a single positive guaic test was reduced by 62% compared with use of three samples in the identification of pathology associated with a positive result (4)

A systematic review revealed, with respect to the diagnosis of all neoplasms (4), that:

• specificity ranged from 65.0% (sensitivity 44.1%) to 99.0% (sensitivity 19.3%) for guaiac FOBTs, and 89.4% (sensitivity 30.3%) to 98.5% (sensitivity 5.4%) for immunochemical FOBTs (5)

Reference:

• 1) British Society of Gastroenterology (2005). Guidelines for the management of iron deficiency anaemia
• 2) Friedman A et al. Use and abuse of faecal occult blood tests in an acute hospital inpatient setting. Internal Medicine Journal 2010; 40(2): 107-111
• 3) Annals of Internal Medicine 1997; 126 (10): 808-810
• 4) Personal communication 8/3/2010. Dr Andrew Harversson. Audit of faecal occult blood testing from 2000-2010 - Kenyon Medical Centres, Coventry.
• 5) Birch JA et al. Diagnostic accuracy of faecal occult blood tests used in screening for colorectal cancer: a systematic review.J Med Screen 2007;14:132-137