[Reaction of unconjugated and conjugated bilirubin with Fast Red TR and related diazonium salts].

In the present study, the reaction of 15 different diazonium salts with serum bilirubin is examined. It is shown that the selectivity of the determination of non-conjugated and total bilirubin greatly depends on two conditions: the pH-value of the reaction mixture and the nature and position of substituents within the benzene ring. Introduction of "electron-pushing" groups such as the o-methyl group of Fast Red TR will change the reactivity of the diazonium cation towards bilirubin in such a way that the co-reaction of the conjugated fraction becomes negligible at pH-values between 2 and 5. After neutralization to pH 7 the total bilirubin can be determined.