Which of the following fractions of bilirubin is water soluble and reacts with a diazo reagent without the addition of an accelerator?

• A. Conjugated bilirubin
• B. Unconjugated bilirubin
• C. Total bilirubin
• D. Indirect bilirubin

Answer: (A)

The main idea is that bilirubin exists as two main forms in the blood: direct (conjugated) bilirubin, which is water soluble, and indirect (unconjugated) bilirubin, which is not. In the diazo bilirubin test, only the water-soluble form reacts directly with the diazotized reagent to form the colored azobilirubin. Since conjugated bilirubin has already been made water soluble in the liver by conjugation with glucuronic acid, it can react with the diazo reagent without any accelerator. Unconjugated bilirubin, being lipid-soluble and largely albumin-bound, does not react directly and typically requires an accelerator to solubilize it before reaction. Therefore, the fraction that is water soluble and reacts with a diazo reagent without an accelerator is the conjugated (direct) bilirubin.