Quantitative effects of bilirubin structural photoisomers on the measurement of direct bilirubin via the vanadate oxidation method

Background: Exposing blood serum samples to ambient white light-emitting diode (WLED) light may accelerate bilirubin photoisomer production. We previously demonstrated the quantitative effect of bilirubin configurational isomers (BCI) on direct bilirubin (DB) value using the vanadate oxidation method. However, the effects of bilirubin structural photoisomers (BSI) remain unclear. Methods: In Study 1, the relationship between WLED irradiation time and BSI production was examined. Serum samples from five neonates were irradiated with WLED light for 0, 10, 30, 60 and 180 min. Bilirubin isomer concentration and BSI production rates were calculated. In Study 2, we performed quantitative investigation of BSI effect on DB values: Differences in DB, BCI and BSI values before and after irradiation were calculated as (sic)DB, (sic)BCI and (sic)BSI, respectively. Assuming the coefficient of BCI affecting DB values was 'a', relational expression was (sic)DB = a*(sic)BSI + 0.19*(sic)BCI. Serum samples from 15 neonates were irradiated with green LED light for 10 and 30 s. The respective bilirubin isomer levels were measured, and the coefficient was derived. Results In Study 1, the median BSI production rate was 0.022 mg/dL per min in specimens with an unconjugated bilirubin concentration of 10.88 mg/dL. In Study 2, assuming that (sic)DB-0.19*(sic)BCI was Y and (sic)BSI was X, the relational expression was Y = 0.34X-0.03 (R-2 = 0.87; p < .01) and a = 0.34. Conclusions Under ambient WLED light, serum sample generated 1.3 mg/dL BSIs in 1 h. Approximately 34% (0.44 mg/dL) of BSI concentrations was measured as DB when using the vanadate oxidation method according to the above equation.