Influence of bilirubin on human platelets

Studies were performed which showed that unbound unconjugated bilirubin at low concentration (0.5 mg/100 ml) causes yellow staining and aggregation of washed human platelets, whereas bilirubin bound to albumin and bilirubin altered by exposure to light have little, if any, of these effects on platelets. Less intense aggregation occurred in some but not all samples of platelet-rich plasma incubated with bilirubin and only at bilirubin levels of 10 mg/100 ml or higher. Aggregation of platelets by bilirubin showed an absolute requirement for calcium ions and this response was enhanced by potassium ions. Adenosine diphosphate and adenosine triphosphate were released from platelets with bilirubin. Speculation: In the early accounts of human kernicterus, and in studies of experimental hyperbilirubinemia in animals, hemorrhagic complications were well known but poorly understood. The data presented suggest that bilirubin influences platelet function. It is tempting to speculate that the effect of bilirubin on platelets could play a role in producing intravascular thrombosis or hemorrhage, or both, in severe hyperbilirubinemia.