1372 PRODUCTION OP BILIRUBIN PHOTOISOMERS (PIB) DURING PHOTOTHERAPY (PT)

Abstract

The Bilirubin Hematofluorometer measures albumin-bound bili-rubin (AB) and total blood bilirubin (TB). A risk factor for developing kernicterus is the level of non-AB. The fraction of TB not bound by albumin is defined as TB-AB/TB=ΔB. In two studies we found that ΔB increased during PT. In 23 jaundiced preterm infants, mean ΔB (ΔB) increased during 4 to 24 hours of phototherapy (Pre-PT, ΔB ± S.D. = 0.15 ± 0.10 and Post-PT, ΔB ± S.D. = 0.24 ± 0.11 = P<0.005, by paired t-test). In a study of 17 jaundiced pre-term infants, ΔB was determined serially during 24 hours of continuous PT; (ΔB) increased by 0.1 ± 0.1 over Pre-PT values at 4 hours (P<0.005) but declined to baseline by 12 to 18 hours. The Photochemical behavior of bilirubin may explain the apparent rise in ΔB during PT. Radiation at 465 nm converts bilirubin to three water soluble PIB. This isomerization is associated with: an increase in absorbance at 470 to 530 nm (max 495 nm); a 22% decline in fluorescence intensity of PIB mixtures at 520 nm (excitation at 465 nm); and an altered circular dich-roic spectrum. We suggest that the increase in ΔB during PT is due to the formation of albumin-bound PIB which fluoresce less intensely than AB and not to an increase in unbound bilirubin. The decline in ΔB during prolonged PT is unexplained. Conclusion: By converting bilirubin to photoisomers, phototherapy may reduce the body burden of neurotoxic bilirubin.