Bilirubin/albumin ratios in fetal blood and in amniotic fluid in rhesus immunization.

OBJECTIVE: To test the hypothesis that unconjugated bilirubin is equally distributed over the albumin molecules present in fetal blood and amniotic fluid in Rhesus (Rh) immunization. METHODS: Molar concentrations of unconjugated bilirubin and albumin were measured in fetal blood and amniotic fluid samples, obtained before the first intrauterine transfusion in 30 nonhydropic, anti-D-alloimmunized fetuses, with gestational ages ranging from 20 to 35 weeks. RESULTS: Bilirubin concentration in amniotic fluid was best predicted by a combination of bilirubin concentration in fetal blood (P<.001), albumin concentration in fetal blood (P=.008), and albumin concentration in amniotic fluid (P<.001) (adjusted R2=0.91). The bilirubin/albumin ratios in fetal blood were linearly correlated with the bilirubin/albumin ratios in amniotic fluid (R2=0.75, P<.001). However, the bilirubin/albumin ratios in fetal blood were always higher than the bilirubin/albumin ratios in amniotic fluid (regression coefficient 1.4, 95% confidence interval 1.1-1.7). In our population, a bilirubin/albumin ratio in amniotic fluid of 0.10 or greater had a better sensitivity and specificity to predict severe anemia (Z hemoglobin -5 standard deviations or less) than the Queenan 4 or the Liley 2c line. CONCLUSION: The relation between fetal hemolysis and amniotic fluid bilirubin concentration is based on the linear correlation between bilirubin/albumin ratios in fetal blood and in amniotic fluid. The slope in Queenan's and Liley's chart follows that of the albumin concentration in amniotic fluid during gestation. LEVEL OF EVIDENCE: III.