False positive or false negative stool guaiac test

Q: What can turn stool guaiac test false positive or false negative?

Answer:

False positive:

• Iron supplements,
• Bismuth containing products
• Red meat (the blood it contains can turn the test positive), 
• Many vegetables particularly if raw like cucumber, cauliflower and horseradish, which contain a chemical with peroxidase properties that can turn the test positive. 

False negative:

• Vitamin C and citrus fruits (which can turn the test falsely negative) 

References:

1. Beg M, et al. (2002). "Occult Gastrointestinal Bleeding: Detection, Interpretation, and Evaluation" JIACM 3 (2): 153–8.
2. Jaffe RM, Kasten B, Young DS, MacLowry JD (December 1975). "False-negative stool occult blood tests caused by ingestion of ascorbic acid (vitamin C)". Ann. Intern. Med. 83 (6): 824–6.
3. Anderson GD, Yuellig TR, Krone RE (May 1990). "An investigation into the effects of oral iron supplementation on in vivo Hemoccult stool testing". Am. J. Gastroenterol. 85 (5): 558–61.
4. Wells HJ, Pagano JF (1977). "Hemoccult test — reversal of false-negative results due to storage". Gastroenterology 72: 1148.