A bilirubin blood test is a laboratory test that measures the level of bilirubin in the blood. Bilirubin is a yellowish substance that is produced when the liver breaks down old red blood cells. It is then transported to the gallbladder and small intestine, where it is excreted in the bile.

The bilirubin blood test is typically ordered as part of a liver panel or a complete blood count (CBC) to assess liver function and to help diagnose and monitor various medical conditions. An elevated level of bilirubin in the blood (hyperbilirubinemia) may indicate liver disease, bile duct obstruction, or other conditions that affect the liver or bile ducts. A low level of bilirubin in the blood (hypobilirubinemia) may be due to certain medications or conditions that affect the liver or bile ducts.

The test is usually performed on a sample of blood that is drawn from a vein in the arm. It can also be performed with a urine test.