The Agappe Bilirubin Direct reagent is used for the quantitative measurement of direct (conjugated) bilirubin in human serum or plasma. It's a diagnostic tool, often part of a larger Bilirubin Total & Direct kit, employed to assess liver function and diagnose conditions like jaundice, liver disease, or biliary obstruction. Direct bilirubin is water-soluble and has been processed by the liver, making this measurement crucial for identifying the cause of liver dysfunction or biliary obstructions.