Which test result is used to detect hemolytic disease of the newborn by amniotic fluid analysis?

• A. Absorbance at 450 nm
• B. Creatinine
• C. Lecithin/sphingomyelin ratio
• D. Estriol

Answer: (A)

The key measurement here is the amount of bilirubin in amniotic fluid. When fetal red cells are destroyed in hemolytic disease of the newborn, unconjugated bilirubin builds up and gives the amniotic fluid a yellow color. Bilirubin absorbs light most strongly at about 450 nm, so spectrophotometric analysis at 450 nm provides a direct and quantitative estimate of bilirubin concentration in the fluid. This absorbance value lets clinicians gauge the severity of the disease (often using a chart like Liley’s) and guide management. The other tests serve different purposes: creatinine reflects fetal kidney function, the lecithin/sphingomyelin ratio assesses fetal lung maturity, and estriol is a hormone not used to diagnose HDN.