Table 1.

Substances That Interfere with Guaiac Tests for Fecal Occult Blood

False-positive results
• Meat (rare or well done)
• Horseradish
• Turnips
• Ferrous sulfate (stool pH<6.0)
• Tomatoes
• Fresh red cherries
False-negative results
• Vitamin C
• Storage of specimen >4 d
• Outdated reagent or card