Fecal Occult Blood
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Guaiac (e.g., Hemoccult) is a reagent that turns blue when oxidized.
A peroxidase catalyst such as that found in blood will trigger the reaction in
the presence of peroxide.  Other catalysts may cause false positives:

þ myoglobin or hemoglobin in meat or fish

þ Turnips, horseradish, bananas, grapes, pears, and plums all contain
  peroxidase

þ (Cimetidine will cause a false positive in gastric aspirate)

False negatives may also occur:

þ Vitamin C suppresses the oxidation reaction, as it is an anti-oxidant.

þ High bulk diets may dilute the blood.

þ In prolonged GI transit time, the peroxidase activity of blood is lost by
  digestion.

  [Connors PJ, Bynum TE.  Is the fecal occult blood test worthwhile?
  Consultant 1991; July:19-23.]

þ a positve Hemoccult test, even if the patient is taking iron, is significant
  for GI bleed.  This is true even though iron preparations applied directly
  to a Hemoccult card will make it turn blue.  [Dig Dis Sci 1988;33:172.]

þ a low pH in gastric aspirate will cause a false negative; thus, the
  buffered Gastrocult card was developed.  A regular Hemoccult card will
  suffice for most ED testing to determine if "coffee grounds" is digested
  blood, though will want to use a Gastroccult for ICU testing for small
  amounts of stress bleeding.  [Roberts JR.  The ins and outs of laboratory
  testing.  Em Med News 1992;14(12):7-9.]

The Hematest brand (orthotolidine) test has many false positives and is
unreliable [Dig Dis 1976;21:845.]