Faecal Occult Blood (FOB) test

Figure 3: Faecal Occult Blood test kit

The FOB test is based upon;

• Blotting paper impregnanted with guaiac
• Rub a small amount of stool on the paper
• Drop a few drops of stabalised
• Blood in stool will act a peroxidase and catalyse the breakdown of the
• The released substance will cause a change in colour of the guaiac to blue

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;

• Vegetable peroxidases
• Meat peroxidases
• Peroxidase activity of some oral iron preparations

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.