Direct bilirubin (conjugated) reacts with the diazonium salt 2,4-dichlorophenyldiazonium (2,4-DPD) in the presence of sulfamic acid to form azo-bilirubin, this coloured complex can be measured photometrically at 546 nm. Of the two bilirubin fractions in serum –bilirubin-glucuronide (conjugated) and free bilirubin which is bound to albumin (unconjugated)– only the first reacts directly, while free bilirubin reacts after being displaced from protein (albumin) by an accelerator. The difference between the two measurements: total bilirubin (with accelerator) and direct bilirubin (without accelerator) enables to calculate indirect bilirubin. The terms «direct» and «indirect» bilirubin refers exclusively to the reaction characteristics in the presence or absence of an accelerator or solubilizer and are only approximate equivalents of the two bilirubin fractions.