Increased Levels of Fecal Protoporphyrin and Guaiac Testing

To the Editor.—  Patients with protoporphyria often have abnormally increased amounts of protoporphyrin in their feces. These same patients are at increased risk for the development of cirrhosis, portal hypertension, and gastrointestinal (GI) tract bleeding from esophageal varices and may have guaiac testing performed to evaluate this possibility.Screening tests for the detection of occult GI tract bleeding are based on an observation of a colored reaction product formed by the rapid oxidation of one of several aromatic diamines (benzidine, diphenylamine, orthotolidine, or guaiac). The oxidation reaction occurs in the presence of oxygen liberated from hydrogen peroxide by the peroxidaselike activity of hemoglobin (Fe⁺²-protoporphyrin globin) or of hematin (Fe⁺³-protoporphyrin) derived from degradation of hemoglobin in the enteric tract.The question of whether increased levels of fecal protoporphyrin could result in false-positive guaiac test results arose in the case of a patient with protoporphyria who had persistently