|
- faecal occult blood test (FOBT) is a well-established community-based screening
tool for the early detection of colorectal cancer
- 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 (1)
- 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 (2)
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 (2)
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 (3)
A systematic review revealed, with respect to the diagnosis of all neoplasms
(3), 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 (4)
Reference:
|
