Can oral iron therapy cause a false positive Hemoccult test (faecal blood test)?/nAccording to FASS

Fråga: Can oral iron therapy cause a false positive Hemoccult test (faecal blood test)?

According to FASS, Erco-Fer (ferrous fumarate) can cause miscolouration, but it is not stated whether the Hemoccult test can be interpreted as falsely positive.

Sammanfattning: A number of studies conclude that oral iron therapy does not cause a false positive Hemoccult test. Therefore, although a false positive reaction cannot in individual cases be excluded, a positive reaction in an individual ingesting therapeutic doses of oral iron should not generally be regarded as a false positive.

Svar: A review of the literature has revealed a number of studies, involving hundreds of subjects, concluding that oral iron does not cause a positive guaiac reaction, the reaction upon which the demonstration of stool blood by Hemomoccult test is based (1). However, based on one study involving only 10 subjects (2), many handbooks conclude that oral iron may produce a false positive guaiac reaction. A number of other studies, including that by Coles and Starnes (3), clearly demonstrate that there is no evidence for false positive guaiac reactions cauced by oral iron therapy. Therefore, any positive Hemoccult stool test found in patients ingesting therapeutic doses of oral iron preparations should not be regarded as a false positive test (3).

As reviewed by McDonnell and Elta (1), the controversy surrounding the subject may partly reflect the assumption that because iron salts readily produce a positive guaiac reaction in vitro, and because 90 per cent of oral iron is eliminated in the stool, oral iron should produce a positive guaiac reaction. The authors also suggest an explantion for the discrepancy between the in vitro and in vivo observations. When an oral iron salt such as ferrous sulphate is dissolved in water, the solution becomes acidic. Ferrous irons in the solution are converted to ferric ions by the hydrogen peroxide in the Hemoccult developer. Ferric ions are then able to oxidise guaiagonic acid to guaiacum blue, producing a positive test result. At pH values above 5, the iron precipitates out as highly insoluble hydrous ferric oxide or ferrous dihydroxide, neither of which produce a positive test. Iron is probably eliminated in the stools as the equally insoluble ferrous sulphide which is not able to produce a positive reaction (1). 1 McDonnel M, Elta G: More on oral iron and the Hemoccult test. N Engl J Med 1989; 321: 1684 2 Lifton LJ, Kreiser J: False-positive stool occult blood tests caused by iron preparations. A controlled study and review of literature. Gastroenterology 1982; 83: 860-863 3 Coles EF, Starnes EC: Use of HemoQuant assays to assess the effect of oral iron preparations on stool Hemoccult tests. Am J Gastroenterol 1991; 86: 1442-1444