What is the basis of the Liley method used to predict the severity of hemolytic disease of the newborn?

The Liley method is primarily focused on measuring the change in optical density at a wavelength of 450 nm to predict the severity of hemolytic disease of the newborn (HDN). This method specifically assesses the presence and concentration of bilirubin in amniotic fluid, which is related to the severity of fetal hemolysis. As fetal red blood cells are broken down, bilirubin is released into the amniotic fluid, and the optical density measurement provides a quantifiable means of determining bilirubin concentrations.

A key aspect of this technique is that it allows clinicians to visualize and gauge the extent of hemolytic conditions affecting the fetus. The correlation between the optical density at this particular wavelength and the degree of fetal blood destruction is significant, which is why this method is relied upon in clinical practice. The optical density measurement enables early intervention and monitoring of risk in pregnancies complicated by Rh incompatibility or other hemolytic conditions.

In contrast, other options provided do not directly measure the bilirubin concentration in the context of HDN prediction. While bilirubin concentration is important, the Liley method's focus is specifically on optical density as a practical and effective measure. Rh determination pertains to understanding blood group incompatibility but doesn't predict hemolysis severity,