Interpretation
In normal health, most of the bilirubin in the plasma is unconjugated. In cholestasis (i.e. impaired bile flow) there is backflow of conjugated bilirubin into the circulation.
The causes of hyperbilirubinaemia can be split into pre-hepatic, hepatic and post-hepatic categories. Measurement of total bilirubin and conjugated bilirubin may help to differentiate between these categories.
– In pre-hepatic jaundice (e.g. haemolysis), the conjugated bilirubin is unlikely to be significantly elevated.
– In hepatic jaundice (e.g. hepatitis), there may be a mixed picture with elevations of both unconjugated and conjugated fractions. Results vary according to the specific underlying cause.
– In post-hepatic jaundice (e.g. cholestasis), the conjugated bilirubin is likely to be predominantly elevated, with a smaller elevation in unconjugated bilirubin also possible.
In neonatal jaundice, an elevated conjugated bilirubin is indicative of an underlying pathological cause (e.g. biliary atresia).
Conjugated bilirubin may be degraded by exposure to light. Specimens should therefore be protected from light.
Test Method
Photometric colour test for the quantitative determination of direct bilirubin on Beckman Coulter AU5822 analyser.
