A diazo-based dry film for determination of total bilirubin in serum

We have prepared a diazo-based dry film for use in determining total serum bilirubin. On a transparent support are a buffered gelatin layer containing a polymeric quaternary amine (the mordant) and a white, reflective spreading layer that contains all of the components necessary for detection of bilirubin. The method is based on the use of dyphylline and Triton X-100 surfactant to dissociate bilirubin from albumin and subsequent reaction of bilirubin with a diazonium salt [4-(N-carboxymethylsulfamyl)benzenediazonium hexafluorophosphate]. In the dry film, unconjugated, mono- and diconjugated, and strongly protein-linked (delta) bilirubin all react with the diazonium salt to produce azo dyes having absorption maxima at about 520 nm. With reflection densitometry and appropriate mathematical transformation, readings and bilirubin concentrations are linearly related to 260 mg/L. Results correlate well with those by the Jendrassik-Grof (Doumas modification) method (slope 0.994, intercept 1.1, correlation coefficient 0.993, Sy X x 4.0), and the method is precise (CV = 10.0% at Cav = 4.1 mg/L, 2.7% at Cav = 24.5 mg/L, 1.2% at Cav = 102 mg/L for patients' samples) and relatively free of interferences.