Quantitative aspects of bilirubin metabolism for hematologists

Techniques now available for the study of plasma disappearance kinetics of isotopically labeled unconjugated bilirubin have led to new insights into the factors that determine the plasma unconjugated bilirubin concentration (BR). This variable can be shown to depend in turn on five other parameters: the total circulating red cell volume (TRVC), the mean corpuscular hemoglobin concentration (MCHC), the mean red cell lifespan (RBCLS), plasma volume (PV), and the hepatic extraction coefficient for unconjugated bilirubin (ke). Of these, three clearly relect varying aspects of erythrokinetics and red cell physiology, while only one is reflective of liver function. It is not surprising, therefore, that knowledgeable interpretations of measurements of the plasma unconjugated bilirubin concentration can provide substantial information of value to the clinical hematologist. In particular, such interpretations can increase the sensitivity of these measurements as a screening test for the presence of hemolysis and provide the earliest indication of changes in red cell survival, as may occur during the therapy of various acquired hemolytic anemias. Furthermore, an understanding of the physiology of bilirubin in plasma substantially enhances the ability of the physician to categorize individual cases of hyperbilirubinemia as being due to hepatic dysfunction, hemolysis, or some combination of both.