Comparison of direct spectrophotometric determinations of bilirubin with candidate reference method in sera of newborns.

Bilirubin was determined by direct spectrophotometry and by the candidate reference method of Doumas et al. (Clin. Chem. (1985) 31; 1789-1798) in a group of more than 230 newborns, and the results were compared. Direct spectrophotometry by Vink et al. (Clin. Chem. (1988) 34; 67-70) gives the following regression equation in comparison with the reference method: y = 0.967x + 6.57 with r = 0.9874. Comparison of direct spectrophotometry at 463 and 582 nm (for the simultaneous determination bilirubin and HbO2) with the reference method gave the following regression equation: y = 0.971x + 8.16 with r = 0.9814. Two methods of direct two-wavelength spectrophotometry showed statistically insignificant differences, using significance levels p = 0.05 and p = 0.01, in comparison with the reference method. In our group of newborns we determined serum HbO2 concentrations from 0.2 g/l (as a 0.025 fractile) to 2.6 g/l (as a 0.975 fractile) with a median value of 0.76 g/l. In a micromodification, the reference method is insensitive to HbO2 in concentrations of up to 5 g/l. Both direct spectrophotometric methods are insensitive to HbO2 concentrations up to 25 g/l. Direct spectrophotometric methods give results identical to those of the reference method and therefore are very suitable for use in neonatology.