It is my understanding that KB test alone can only tell you if there are fetal cells circulating in the mom and the percentage. It does not tell these circulating fetal cells are Rh- or Rh+. If fetal is Rh-, no matter how many fetal cells circulating, the mom is not indicated for RhoIg. Therefore, KB test itself almost has no role to play in administration of RhoIg, assuming fetal Rh status is unknown. Some may argue that, considering almost 85% population is Rh pos, even though fetal Rh unknown, it may be statistically safe to assume the fetal is Rh+ and just give the RhoIg based on the KB result. Well, it is really a hard sell, especially when there is alternative way to proceed. I will say, in the case of fetal Rh unknown, especially for prenatal mom at ER, first we do fetal screen. If neg, at least you are sure that, regardless the fetal Rh+or -, one shot is enough. if fetal screen is pos, you will automatically give one shot first, then rely on the KB result to determine how many extra shots to give with confidence, because you know the fetal is Rh+ based on post fetal screen result. The positive fetal screen tells you the fetal is Rh+, for sure, assuming mom DAT is negative (otherwise may cause false pos fetal screen). Therefore I recommend the combination of fetal screen and KB test. Whether to do KB test depends on the fetal screen result first. I still do not understand what is the reason to let Ortho limit fetal screen kit in the postpartum with known Rh of newborn (Rh+, based on Cord or heelstick grouping test)and the mom (Rh-) in its product insert.