False-positive stool occult blood tests caused by iron preparations. A controlled study and review of literature

Stools from 10 male volunteers were tested by Hemoccult and Hematest occult blood methods after the ingestion of ferrous sulfate and ferrous gluconate. The volunteers served as their own controls. The Hemoccult methods had 65% and 50% false-positive reactions while the Hematest had 25% and 65% false-positive reactions after ferrous sulfate and ferrous gluconate, respectively. In vitro testing confirmed that ferrous compounds caused a false-positive Hemoccult slide reaction. Previous studies of the effect of medicinal iron on occult blood tests are reviewed.