Conjugated bilirubin

Conjugated or direct hyperbilirubinaemia occurs when the the liver is able to conjugate bilirubin, but the excretion is impaired.

Causes include:

• failure of bilirubin excretion by hepatocytes:
  • Dubin-Johnson syndrome
  • Rotor's syndrome
• obstruction to biliary flow i.e. cholestasis, both intra-hepatic and extra-hepatic

The proportion of conjugated bilirubin with respect to the total raised bilirubin can give a clue as to cause of jaundice.

20-40% of total: more suggestive of hepatic than posthepatic jaundice

40-60% of total: occurs in either hepatic or posthepatic causes

> 50% of total: more suggestive of posthepatic than hepatic jaundice

Note that if conjugated bilirubin is less than 20% of total then the cause of hyperbilirubinaemia is secondary to haemolysis or constitutional e.g. Gilbert's disease, Crigler-Najjar syndrome