Patients should consult their physician about any preparatory steps that may be necessary to ensure the success of their fecal occult blood test (FOBT). Various foods, medications and supplements can cause false positive or negative results during testing.

For example, aspirin use can cause inflammation of the stomach (gastritis) and subsequent bleeding, which could then lead to a positive FOBT even though the source of the blood is not due to a tumor. Conversely, taking a vitamin C supplement could suppress the mechanism used in the test and create a false negative result. In this case, cancer would be present, but not detected by the FOBT.

Patients should follow their physician’s advice, and should never stop taking any medication without their physician’s approval. As a general rule, patients should avoid the following for three days (seven days in the case of NSAIDs) prior to testing:

• Nonsteroidal anti-inflammatory drugs (NSAIDs), including ibuprofen and aspirin (no more than one aspirin a day)
  
  
• Anticoagulants (which reduce the blood's ability to clot)
  
  
• Vitamin C from supplements or citrus fruits in excess of 250 milligrams daily
  
  
• Red meat (beef or lamb)
  
  
• Raw fruits and vegetables, especially melons, radishes, turnips and horseradish

In addition, the test should not be completed if the patient has certain medical conditions, such as bleeding hemorrhoids. With women, the test should not be done during menstruation.