The bilirubin blood test measures the levels of bilirubin in the bloodstream. Bilirubin is a yellowish pigment that is produced when red blood cells are broken down in the body. It is processed by the liver and excreted in the bile. The bilirubin blood test is often used to assess liver function and diagnose conditions that affect the liver or bile ducts. It can help determine if there is an excess of bilirubin in the blood, which may indicate liver disease, bile duct obstruction, or other conditions that affect the normal processing and excretion of bilirubin.