Obstructive jaundice of the newborn is a misnomer as fewer than half of all cases usually referred to as "obstructive jaundice" result from mechanical interference with the flow of bile. In the majority of cases, intrinsic liver disease is the cause of this type of jaundice. The term "cholestasis" more comprehensively describes the various disorders and the pathophysiology, which includes retention of direct-reacting (conjugated) bilirubin, bile acids, and other components of bile.
Direct-reacting hyperbilirubinemia is almost always the first definitive sign of hepatobiliary dysfunction, often appearing during the first weeks of life just as the normally occurring, indirect-reacting (unconjugated) hyperbilirubinemia of physiologic jaundice of the newborn is receding. Initially, there may be uncertainty as to the significance of the persisting or recurring jaundice, particularly whether the direct-reacting fraction is elevated.
As a general rule, when the direct-reacting portion exceeds 2.0 mg/dl and is more than 10% of the total serum bilirubin, it should be considered clinically significant. Although the direct-reacting fraction may occasionally account for 90% of total serum bilirubin, in most cases the direct-reacting portion will range only from 25% to 75%. The degree of variation has no diagnostic significance. As only water-soluble, direct-reacting bilirubin is excreted by the kidney, the presence of bilirubinuria may be used as confirmation of elevation of the direct-reacting fraction.