Bilirubin is absent in urine because it is :

Unconjugated serum bilirubin is always bound to albumin, is not filtered by the kidney (not found in the urine) In biliary obstruction or hepatocellular diseases, both conjugated and unconjugated bilirubin accumulate in plasma. In hemolytic jaundice, total plasma bilirubin increases, but the propoion of the unconjugated and conjugated fractions remains unchanged.