Light has a greater effect on direct bilirubin measured by the bilirubin oxidase method than by the diazo method.

We compared the effect of light on direct-reacting bilirubin (DBIL) measurement by the bilirubin oxidase (EC 1.3.3.5; BOX) method and by the Jendrassik-Gróf diazo method. DBIL concentrations determined by the BOX method in the sera of hyperbilirubinemic infants treated with phototherapy yielded falsely higher values than those by the direct diazo method. A similar tendency was noted when DBIL concentrations in infants' sera irradiated with light in vitro were determined by both methods, although by HPLC none of these sera had detectable DBIL (i.e., conjugated plus delta bilirubin). In general, DBIL concentrations after photoirradiation remained unchanged when measured by the diazo method, but significantly increased when the BOX method was used. Indeed, photoirradiation gave rise to material that acted like a photobilirubin product, which was oxidized at pH 3.7 and therefore was measured as DBIL. Such false increases in DBIL values generated by the BOX method may have clinical diagnostic implications in monitoring jaundiced neonates and in differentiating between physiological jaundice and incipient pathological jaundice.