TOTAL and DIRECT
Bilirubin is converted to coloured azobilirubin by diazotized sulfanilic acid and is measured photometrically. Of the two bilirubin fractions in serum –bilirubin-glucuronide and free bilirubin which is bound to albumin– only the former reacts directly, while free albumin reacts after being displaced from protein by an accelerator. The difference of two measurements total bilirubin (with accelerator) and direct bilirubin (without accelerator) enables the calculation of 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.