MEASUREMENT OF METHEMOGLOBIN IN NEONATAL SAMPLES CONTAINING FETAL HEMOGLOBIN

Because of the potential for methemoglobinemia during nitric oxide the rapy in newborns, methods are needed to accurately quantify methemoglo bin (MetHb) in the presence of the high concentrations of fetal hemogl obin (Hb Fl, bilirubin, and lipids seen in these patients. Spectral di fferences between fetal and adult Hbs invalidate assumptions of conven tional multiwavelength Hb photometry, so we evaluated an ''overdetermi ned'' system (Ciba-Corning Model 270), in which absorbances at seven w avelengths are measured to quantify four Hb derivatives. Adult and umb ilical cord blood (Hb F 96%) samples were prepared to contain known Me tHb fractions. Measured MetHb was linear in cord blood to greater than or equal to 15% MetHb. Within-run precision (CV) was <2.2% (n = 10) a t each of seven MetHb fractions between 5% and 100%. Measured (y) and expected (y) MetHb fractions in cord blood were in good agreement (y = 1.0200x + 0.100, S-ylx = 0). Added bilibrubin (200 mg/L serum) and li pid (30 g/L) did not interfere. No significant differences were seen f or adult and cord blood samples with identical MetHb fractions (P = 0. 72), whereas a significant difference was noted with an exactly determ ined system (P = 0.0033). At clinically relevant MetHb fractions (<15% ), a trend towards increased values in cord blood was noted with an ex actly determined system (y = 1.0520x + 0.7600). We conclude that this overdetermined system measures MetHb accurately in samples from patien ts with large concentrations of Hb F.