Bilirubin is derived from the haemoglobin of aged or damaged red blood cells. Bilirubin does not contain iron, but is rather a derivative of the heme group. Some part of serum bilirubin is carried in the plasma to the liver, where it is conjugated to form bilirubin diglucuronide and excreted in the bile. Conjugated bilirubin is also referred “direct bilirubin” because of the fact it reacts directly with the analytical reagent without any solubilizing agent. (The unconjugated bilirubin is referred to as indirect bilirubin. Total bilirubin includes both the conjugated and unconjugated (free) forms).

Serum bilirubin concentration reflects the ability of hepatocytes to take up, conjugate and secrete bilirubin, so it is a functional marker rather than a marker of cellular integrity as reflected by serum transaminase levels. Pathologic elevation of conjugated bilirubin (concentration higher than 2 mg/dL or more than 20% of total bilirubin) is termed conjugated hyperbilirubinemia. It is a biochemical marker of cholestasis and hepatocellular dysfunction. Moreover, it may be observed in systemic illnesses with hepatic involvement. 

Serum bilirubin levels are used to rule out clinical conditions:

Pre-hepatic        -   Haemolytic crisis

Hepatic                 - Hepatic disease (decreased ability to conjugate and excrete bilirubin), intrahepatic cholestatic disease

Post-hepatic      -   Bile duct obstruction results in an accumulation of conjugated bilirubin.