Bilirubin is an orange-yellow pigment, a waste product primarily produced by the normal breakdown of heme. Heme is a component of hemoglobin, which is found in red blood cells (RBCs). Bilirubin is ultimately processed by the liver to allow its elimination from the body. This test measures the amount of bilirubin in the blood to evaluate a person’s liver function or to help diagnose anemias caused by RBC destruction (hemolytic anemia).
RBCs normally degrade after about 120 days in circulation. As heme is released from hemoglobin, it is converted to bilirubin. This form of bilirubin is also called unconjugated bilirubin. Unconjugated bilirubin is carried by proteins to the liver; there, sugars are attached (conjugated) to bilirubin to form conjugated bilirubin. Conjugated bilirubin enters the bile and passes from the liver to the small intestines; there, it is further broken down by bacteria and eventually eliminated in the stool. Thus, the breakdown products of bilirubin give stool its characteristic brown color.
A small amount (approximately 250 to 350 milligrams) of bilirubin is produced daily in a normal, healthy adult. Most (85%) of bilirubin is derived from damaged or degraded RBCs, with the remaining amount derived from the bone marrow or liver. Normally, small amounts of unconjugated bilirubin are released into the blood, but virtually no conjugated bilirubin is present. Both forms can be measured or estimated by laboratory tests, and a total bilirubin result (a sum of these) may also be reported.
If the bilirubin level increases in the blood, a person may appear jaundiced, with a yellowing of the skin and/or whites of the eyes. The pattern of bilirubin test results can give the health practitioner information regarding the condition that may be present. For example, unconjugated bilirubin may be increased when there is an unusual amount of RBC destruction (hemolysis) or when the liver is unable to process bilirubin (i.e., with liver diseases such as cirrhosis or inherited problems). Conversely, conjugated bilirubin can increase when the liver is able to process bilirubin but is not able to pass the conjugated bilirubin to the bile for removal; when this happens, the cause is often acute hepatitis or blockage of the bile ducts.
Increased total and unconjugated bilirubin levels are relatively common in newborns in the first few days after birth. This finding is called “physiologic jaundice of the newborn” and occurs because the newborn’s liver is not mature enough to process bilirubin yet. Usually, physiologic jaundice of the newborn resolves itself within a few days. However, in hemolytic disease of the newborn, RBCs may be destroyed because of blood incompatibilities between the baby and the mother; in these cases, treatment may be required because high levels of unconjugated bilirubin can damage the newborn’s brain.
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