Bilirubin Metabolism and Jaundice

Key points

• • Old red cells are broken down by macrophages; haem becomes biliverdin and then unconjugated bilirubin, which is fat-soluble and travels in blood bound to albumin.
• • The liver conjugates bilirubin with glucuronic acid using UDP-glucuronyl transferase, making it water-soluble so it can be excreted in bile.
• • Gut bacteria convert conjugated bilirubin to urobilinogen; some becomes stercobilin (brown stool) and some is reabsorbed and excreted in urine.
• • Jaundice generally becomes clinically visible once serum bilirubin reaches roughly 2 to 3 milligrams per decilitre, about twice the upper limit of normal, staining the sclera and skin yellow.
• • Pre-hepatic (haemolytic) jaundice raises unconjugated bilirubin; the urine contains no bilirubin but more urobilinogen.
• • Post-hepatic (obstructive) jaundice raises conjugated bilirubin with dark urine and pale stools; hepatocellular jaundice typically gives a mixed rise in conjugated and unconjugated bilirubin.