Measuring Free Bilirubin: The Clinical Perspective

Abstract

In 1959, Gerald Odell first proposed that albumin and tissue compete for binding bilirubin and that this competition is mediated by a very low concentration of free bilirubin (Bf)2 (bilirubin not bound to plasma proteins). He argued that displacement of albumin-bound bilirubin by binding competitors could explain the reported high incidence of kernicterus in premature infants receiving sulfonamide prophylaxis (1). This sentinel publication led to a flurry of proposed binding tests, most of which were nonspecific competitive-binding assays, until 1974, when Jacobsen and Wennberg presented a method for measuring Bf in infant plasma that was based on the observation that only unbound bilirubin serves as a substrate for enzymatic oxidation by horseradish peroxidase (2). Several studies from Japan, where the peroxidase assay had been adapted to a semiautomated instrument, identified a clear relationship of Bf with acute neurotoxicity in premature infants (3) and with acute auditory impairment in term infants (4). Enthusiasm was tempered in the US by a perceived difficulty in performing the assay and by confusion regarding the poor correlation between Bf and outcome in premature babies who died with kernicterus at very low total bilirubin (Bt) concentrations (5). The reported high incidence of death of infants with kernicterus was isolated to specific care centers: Very high Bf/Bt ratios in affected infants indicated very poor binding, and the phenomenon of lethal kernicterus at low Bt concentrations largely disappeared after cessation of benzyl alcohol use as a preservative in multiple-dose vials or solutions for parenteral therapy (6). Kernicterus is still occasionally observed at low Bt concentrations in babies with sepsis or …