Bilirubin

A bilirubin test measures the amount of bilirubin in a blood sample. Bilirubin is a brownish yellow substance found in bile. It is produced when the liver breaks down old red blood cells. Bilirubin is then removed from the body through the stool (feces) and gives stool its normal brown color.

Bilirubin circulates in the bloodstream in two forms:

• Indirect (or unconjugated) bilirubin. This form of bilirubin does not dissolve in water (it is insoluble). Indirect bilirubin travels through the bloodstream to the liver, where it is changed into a soluble form (direct or conjugated).
• Direct (or conjugated) bilirubin. Direct bilirubin dissolves in water (it is soluble) and is made by the liver from indirect bilirubin.

Total bilirubin and direct bilirubin levels are measured directly in the blood, whereas indirect bilirubin levels are derived from the total and direct bilirubin measurements.

When bilirubin levels are high, the skin and whites of the eyes may appear yellow (jaundice). Jaundice may be caused by liver disease (hepatitis), blood disorders (hemolytic anemia), or blockage of the tubes (bile ducts) that allow bile to pass from the liver to the small intestine.

Too much bilirubin (hyperbilirubinemia) in a newborn baby can cause brain damage (kernicterus), hearing loss, problems with the muscles that move the eye, physical abnormalities, and even death. Therefore, some babies who develop jaundice may be treated with special lights (phototherapy ) or a blood transfusion to lower their bilirubin levels.

In a newborn baby, the blood sample is usually taken from the heel (heel stick).