Bilirubin is converted to colored azobilirubin by diazotized sulfanilic acid and

measured photometrically. Of the two fractions presents in serum, bilirubin-
glucuromide and free Bilirubin loosely bound to albumin, only the former reacts

directly in aqueous solution (Bilirubin direct), while free Bilirubin requires
solubilization with dimethylsulphoxide (DMSO) to react (Bilirubin indirect). In the
determination of indirect Bilirubin the direct is also determined, the results
correspond to total Bilirubin.
The intensity of the color formed is proportional to the bilirrubin concentration in
the sample