The Kleihauer Betke Test:
A peripheral blood smear of the mom’s blood is prepared and fixed in 80% ethanol. It is then treated with acid (McIlvaine’s buffer) and stained with eosin and hematoxylin. Fetal cells will remain in tact and stain bright pink because of the high HbF/fetal hemoglobin content which resists acid elution. Adult hemoglobin in maternal cells will elute out, leaving faintly staining “ghost cells”. The percental of fetal cells per 2000 cells is used to determine how much extra WinRho (in 300mg vials) is required:
Vials = %fetal cells x maternal blood volume (~5000mL)/30mL
Rounded to the nearest whole number + 1 extra. But that one extra is usually the shot they automatically gave her after she gave birth.