Chemical versus Spectrophotometric Determination of Bilirubin in Amniotic Fluid, and the Influence of Hemoglobin and Metheme Pigments

Abstract

Abstract Methemoglobin and methemalbumin markedly diminish the measurements by spectrophotometry of the change in absorbance at 450 nm (ΔA450) in amniotic fluid but do not affect the chemical (diazo) determination of bilirubin. The converse was true of hemoglobin. In 72 of 82 specimens there was no gross contamination of amniotic fluid with these pigments and the two types of bilirubin determination agreed well (r = 0.98, p<0.001) when a slight modification of the Malloy-Evelyn method was used (1 volume of amniotic fluid was diluted to 3 volumes before addition of reagents). In the other 10 cases the presence of metheme pigments significantly decreased the ΔA450, but in none of the 82 specimens was there sufficient free hemoglobin present to depress significantly the results of the chemical determination, although it can do so if enough is present.