An ortho-tolidine method for the detection of occult blood in faeces.

Benzidine is still the most commonly used reagent for occult blood tests in faeces. About three years ago, however, the manufacture of AnalaR benzidine in this country was discontinued as it was shown that benzidine has very marked carcinogenic properties, particularly in some of the stages of purification to AnalaR specification. This did not appear to be widely recognized probably because most laboratories still had stocks of the AnalaR benzidine. We drew attention to this state of affairs two years ago (Kohn and O'Kelly, 1952). Since then it has been found that the benzidine available is not sufficiently reliable for occult blood testing even when various methods of purification were tried. This affects, of course, any benzidine method whatever the technique. A review of other reagents which could replace benzidine showed that ortho-tolidine had been used both for urine and for faecal occult blood tests (Zwarenstein, 1949; Hawk, Oser, and Sum-