Amniotic fluid bilirubin is increased in association with the severity of hemolytic diseases of the...

Amniotic fluid bilirubin is increased in association with the severity of hemolytic diseases of the newborn. As red blood cells lyse during these conditions, bilirubin builds up as a byproduct of the red cell destruction. The more red blood cells that are being destroyed in the baby, the more increased the bilirubin level will become.

Which one of the following tests BEST correlates with the severity of hemolytic disease of the newborn (HDN).