Bilirubin testing checks for levels of bilirubin in your blood. Bilirubin (bil-ih-ROO-bin), an orange-yellow pigment, is a waste product of the normal breakdown of red blood cells. Bilirubin passes through the liver and eventually out of the body — mostly in feces, a small amount in urine.
Before reaching the liver, bilirubin is called unconjugated, meaning uncombined. In the liver, bilirubin combines with certain sugars to create a water-soluble form called conjugated bilirubin. Conjugated bilirubin passes out of the liver, and in the colon, it is converted back into the unconjugated form en route to being excreted from the body.
Most laboratories use a test that detects conjugated bilirubin, which is called direct. By subtracting the direct bilirubin from the amount of total bilirubin, an estimate of unconjugated bilirubin, called indirect, is obtained.
Higher than normal levels of direct or indirect bilirubin may indicate different types of liver problems. Occasionally, higher bilirubin levels may indicate an increased rate of destruction of red blood cells (hemolysis).