Abstract: A reagent composition, dry analytical element and a method of using same to determine total bilirubin in aqueous liquids are disclosed. The reagent composition comprises a diazonium salt, or reagents capable of producing a diazonium salt when contacted with water, and 3,3-dimethylglutaric acid or a salt thereof present in an amount effective to buffer the composition at a pH of about 3.5 or less when the composition is contacted with a 2 .mu.L sample of pooled human serum. The composition and elements described herein provide a means to eliminate the effect of an undetermined interferent which is either preformed or formed in situ in diazo-based total bilirubin determinations of biological fluids (e.g. uremic serum).