••• Chapter 20^ Rh Alloimmunization^209
❍ How is anti-D immune globulin obtained?
It is collected by apheresis from RhD-negative male volunteer donors who are given multiple injections of
RhD-positive red cells and thus have high titers of circulating anti-RhD antibodies. A search is currently
underway for a synthetic anti-D immune globulin and some progress has been made on this front.
❍ Is Rh immune globulin indicated when a patient is already sensitized?
No.
❍ How large a fetomaternal hemorrhage does the 300 mg dose of Rh immune globulin dose cover?
30 mL of fetal whole blood or 15 mL of fetal red cells (only 1% of women have >5 mL of fetal blood in the
maternal circulation after delivery).
❍ What test is used to QUANTITATE the volume of fetal red cells in the maternal circulation?
Kleihauer-Betke (KB).
❍ What is the QUALITATIVE test for fetomaternal hemorrhage?
Rosette test. If this test is negative, a standard dose of Rh immune globulin should be given. If this test is
positive, further evaluation is recommended using the KB test to evaluate the percentage of fetal cells in maternal
circulation.
❍ How is the dose of Rh immune globulin calculated if the volume of hemorrhage is estimated to be >30 mL
of whole blood?
When fetomaternal hemorrhage is >30 mL of whole blood, the KB is performed to quantitate the actual volume
of hemorrhage. The percentage of fetal cells is multiplied by 50 (to account for estimated maternal blood volume
of 5 L). This number is divided by 30 to determine the number of vials of Rh immune globulin (because each
vial of 300 mg of Rh immune globulin neutralizes 30 mL of fetal whole blood). One additional vial of Rh
immune globulin is always added to the number above as a precaution.
❍ Within what time limit should Rh immune globulin be given after delivery?
The standard time interval for Rh immune globulin to be given is within 72 hours; however, this is just a
by-product of how the original studies were performed because women had to return within 3 days. Rh immune
globulin should be given before a primary immune response occurs, and it can be given up to 14 to 28 days
after delivery.
❍ How long does the effect of Rh immune globulin last?
The half-life of Rh immune globulin is 24 days, and a woman can be considered fully protected for 12 weeks after
injection. There are scattered case reports of maternal sensitization from decreasing antibody concentrations.
❍ If standard antenatal anti-D immune globulin administration is given within 3 weeks of delivery, can the
postnatal dose be withheld in the absence of excessive fetomaternal hemorrhage?
Yes.
❍ What percentage of women have evidence of fetomaternal hemorrhage after delivery?
75%.