What type of bilirubin is referred to as conjugated?

Bilirubin is a byproduct of the breakdown of hemoglobin from red blood cells. It exists in two primary forms: conjugated and unconjugated.

Conjugated bilirubin, also known as direct bilirubin, is the form of bilirubin that has been processed by the liver. In this process, bilirubin that is initially produced from hemoglobin breakdown (unconjugated or indirect bilirubin) is taken up by liver cells and made water-soluble through a chemical reaction with glucuronic acid. This transformation allows the kidney and intestines to excrete bilirubin effectively, which is crucial for eliminating it from the body.

Direct bilirubin can be measured in blood tests and indicates that the liver is functioning normally in processing bilirubin. Elevated levels of conjugated bilirubin can indicate liver dysfunction or bile duct obstruction, leading to conditions such as cholestasis or hepatitis.

In contrast, the other forms of bilirubin listed—indirect bilirubin, unconjugated bilirubin, and free bilirubin—refer to the bilirubin that has not yet undergone this conjugation process and can typically signify different clinical conditions when elevated. Understanding the distinction between these forms is vital in diagnosing and managing liver and hematologic issues.