The FOB test is based upon;
A small sample of stool to be tested is smeared onto the two sample areas. Care is taken not to apply any stool to the positive and negative control areas. The paper smeared with the stool is covered and the card turned over. The window on that side is opened up and the stabalised is dropped onto the paper over the stool samples and over the positive and negative control dots. If there is increased peroxidase activity in the stool sample the is broken down and the guaiac turns blue.
False postive FOB tests may be seen when there is increased peroxidase activity in the stool that is not due to haem;
False negative FOB may be seen if the patient is consuming a large amount of vitamin C (ascorbic acid).
The use of guaiac as the indicator has been critisised as being too insensitive. However if the test is too sensitive it is necessary to eat a meat free diet for three days prior to testing to cut out false positives from meat peroxidases.