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Bilirubin

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Also known as: Total bilirubin; TBIL; Neonatal bilirubin; Direct bilirubin; Conjugated bilirubin; Indirect bilirubin; Unconjugated bilirubin
Formal name: Bilirubin - blood

The Test Sample

What is being tested?

Bilirubin is an orange-yellow pigment, a waste product primarily produced by the normal breakdown of heme, a substance found mainly in the protein hemoglobin in red blood cells (RBCs). It is ultimately processed by the liver to allow its elimination from the body. This test measures the amount of bilirubin in the blood in order to evaluate liver function or to help diagnose anemia caused by the increased destruction of RBCs (hemolytic anemia).

RBCs normally degrade after about 120 days in the circulation. As the heme in hemoglobin is broken down, it is converted into bilirubin; this form is also called unconjugated bilirubin. Unconjugated bilirubin is not very soluble in water, so it is carried by proteins to the liver, where sugars are attached (conjugated) to it to form water-soluble conjugated bilirubin. This conjugated bilirubin enters the bile and passes from the liver to the small intestines, where it is further broken down by bacteria and eventually eliminated in the stool. The breakdown products of bilirubin give stool its characteristic brown color.Drawing of the biliary system

Approximately 250 to 350 mg of bilirubin is produced daily in a normal, healthy adult, of which 85% is derived from damaged or degraded red cells, with the remaining amount 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 (a sum of these) may also be reported. The pattern of results from these tests can give the doctor information as to the condition that may be present.

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. Elevated unconjugated bilirubin may mean that too much is being produced, usually due to increased destruction (hemolysis) of RBCs, or that the liver is incapable of processing bilirubin due to liver diseases such as cirrhosis or inherited problems. Elevated conjugated bilirubin may indicate that the liver can process bilirubin but it is not adequately removed in a timely manner due to acute hepatitis or blockage of bile ducts.

It is not uncommon to see high total and unconjugated bilirubin levels in newborns, typically 1 to 3 days old. This is sometimes called physiologic jaundice of the newborn and is due to immaturity of the newborn's liver. This situation usually resolves itself within a few days. In hemolytic disease of the newborn, RBCs may be destroyed because of blood incompatibilities between the baby and the mother, and treatment may be needed because high levels of unconjugated bilirubin can cause damage to the newborn's brain.

A rare (about 1 in 10,000 births) but life-threatening congenital condition called biliary atresia can cause increased total and conjugated bilirubin levels in newborns and must be quickly detected and treated, usually with surgery, to prevent serious liver damage (cirrhosis) that may require liver transplantation within the first year or two of life. Even after surgery, some children still may require liver transplantation.

How is the sample collected for testing?

For adults, blood is typically collected by needle from a vein in the arm. In newborns, blood is often collected from a heelstick, a technique that uses a small, sharp blade to cut the skin on the infant's heel and collect a few drops of blood into a small tube. Non-invasive technology is available in some health care facilities that will measure bilirubin by using an instrument placed on the skin (transcutaneous bilirubin meter).

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

Fasting requirements vary by laboratory. Ask your lab or health care provider for instructions. You may need to fast (nothing but water) for several hours before the test.