Direct bilirubin is the water-soluble fraction produced after conjugation.

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

Answer: (A)

Direct bilirubin is the conjugated, water-soluble form of bilirubin. In the liver, bilirubin that comes from heme breakdown is initially unconjugated (indirect) and binds to albumin to travel to the liver. There it is conjugated with glucuronic acid, forming a water-soluble compound that is excreted into bile. This conjugated form is what the body refers to as direct bilirubin. The total bilirubin in blood is the sum of direct (conjugated) and indirect (unconjugated) fractions, with indirect being the lipid-soluble form not yet conjugated.