Delta bilirubin is described as which of the following statements?

• A. Conjugated bilirubin bound to albumin
• B. Unconjugated bilirubin bound to albumin
• C. Unconjugated bilirubin covalently bound to albumin
• D. Conjugated bilirubin covalently bound to albumin

Answer: (C)

Delta bilirubin is a bilirubin species that stays in the blood because it is covalently bound to albumin. This covalent attachment makes the complex very stable and long-lasting, so bilirubin can remain elevated for a long time even after the liver has stopped producing or excreting bilirubin at high rates. The bilirubin involved in this complex is the unconjugated form, not the glucuronidated (conjugated) form, which is why the description that fits best is unconjugated bilirubin covalently bound to albumin. This covalent binding is what distinguishes delta bilirubin from the usual bilirubin-albumin association, which is noncovalent and shorter-lived. Clinically, delta bilirubin accounts for persistent, low-level hyperbilirubinemia and can influence how bilirubin fractions appear on tests for some time after an acute liver or biliary issue resolves.