Direct bilirubinConjugated bilirubin
The portion of bilirubin the liver has already processed, used to locate a problem.
Direct, or conjugated, bilirubin is the fraction the liver has already attached a sugar group to, making it water-soluble and ready to leave in bile. The rest, indirect bilirubin, has not yet been through that step.
It matters because the split localises the problem. A rise in the indirect portion suggests red cells being broken down faster than the liver can keep up with, while a rise in the direct portion suggests the processed pigment is not draining, pointing at the bile ducts or the liver cells themselves.
In practice the direct value is read as a proportion of the total rather than alone, and alongside alkaline phosphatase and gamma GT, which also rise when bile flow is obstructed.