A bilirubin blood test measures the amount of bilirubin in your blood. Bilirubin is a yellowish substance your body makes during the normal breakdown of old red blood cells. It travels to your liver, which mixes it into bile and clears most of it out through the bile ducts and stool. Labs can report it in two forms: total bilirubin (all of it added together) and direct (conjugated) bilirubin (the portion already processed by the liver); total minus direct gives the indirect (unconjugated) fraction that has not yet been through the liver. Because bilirubin is produced by red-cell breakdown and cleared by the liver, its level reflects how well both systems are working.
A bilirubin test may be used to check the health of your liver, to look for the cause of jaundice (yellowing of the skin or the whites of the eyes), or to see how well a treatment is working. A clinician often orders it when you have symptoms such as jaundice, dark urine, pale- or clay-colored stool, or stomach/belly pain. Splitting total into direct versus indirect helps distinguish problems before the liver (such as faster red-cell breakdown) from problems in or after the liver (such as a blocked bile duct). It is also commonly used to help diagnose newborn jaundice, since many healthy babies develop jaundice in their first days because their livers have not matured enough to clear bilirubin efficiently.
Reference ranges vary from one lab to another and can differ by age and testing method, so always compare your result to the range printed on your own report. As a general adult example, MedlinePlus lists direct bilirubin as less than 0.3 mg/dL (less than 5.1 µmol/L) and total bilirubin as 0.1 to 1.2 mg/dL (1.71 to 20.5 µmol/L). MedlinePlus explicitly notes that "normal value ranges may vary slightly among different labs." Newborns normally have much higher levels and are judged against separate age- and hour-specific thresholds, not the adult range.
A higher-than-normal level (hyperbilirubinemia) is a signal to investigate, not a diagnosis by itself, and must be interpreted by a clinician alongside your symptoms, exam, and other tests. It may be due to increased red-blood-cell breakdown (hemolysis), such as hemolytic anemia, transfusion reactions, or, in newborns, erythroblastosis fetalis; liver problems such as hepatitis, cirrhosis, other liver disease, or Gilbert disease (a common, usually harmless inherited condition); or a blockage in the bile ducts or gallbladder, such as gallstones, a biliary stricture, or, less commonly, pancreatic, gallbladder, or bile-duct cancers. Which fraction is elevated (direct versus indirect) helps narrow down the cause.
Low bilirubin levels (hypobilirubinemia) are usually not a cause for concern. When a low value does have a cause, it may be related to certain medicines, for example some antibiotics, birth control pills, sleeping pills, and seizure medicines. A low result on its own is generally not treated as a health problem, but any specific concern should be discussed with your clinician.
Reference ranges vary by laboratory, age, sex, and method — the range on your own report is what applies to you. A single value out of range doesn’t confirm any condition; your clinician interprets it alongside your symptoms, history, and other results. This page is general information, not medical advice.
General reference, not medical advice, and not a substitute for your clinician. Lab reference ranges and interpretation depend on the laboratory and on your individual situation — discuss your results with a licensed healthcare professional.