A bilirubin test checks liver health by measuring the levels of bilirubin in the blood. Bilirubin is a substance produced by the breakdown of red blood cells.Bilirubin (bil-ih-ROO-bin) passes through the liver and is eventually excreted out of the body.Higher than usual levels of bilirubin may mean different types of liver or bile duct problems. Sometimes, higher bilirubin levels may be caused by an increased rate of destruction of red blood cells.