bilirubin blood test, a blood test performed in cases of jaundice to help determine whether the jaundice is caused by hepatocellular dysfunction (as in hepatitis) or extrahepatic obstruction of the bile ducts (as with gallstones or tumor blocking the bile ducts). Total serum bilirubin is made up of conjugated (direct) and unconjugated (indirect) bilirubin, with varying ratios of each characterizing different diseases.
