One of the main purposes of perinatal testing programs is to determine which patients are candidates for RhIG. If given soon enough following exposure to D+ red cells, and in a suitable dosage, RhIG has the ability to prevent immunization to D.
RhIG products are available in a standard dose of 300 µg (1500 IU) and a small (micro) dose of 50 µg (250 IU). Note: 1 µg of anti-D = 5 IU. The 50 µg dose is used for Rh-negative patients whose pregnancy terminates in the first 12 weeks of pregnancy. After 12 weeks a full dose (300 µg) is administered when there is a similar risk of fetomaternal hemorrhage (FMH). A full dose of RhIG protects against a bleed of 30 mL of Rh-positive fetal whole blood (15 mL packed cells).
RhIG should be administered within 72 hours of any event that would be associated with a risk for FMH, such as amniocentesis or delivery; however, RhIG should not be withheld if this time limit has been exceeded. These dosages are recommended:
- United States: American College of Obstetricians and Gynecologists (2017): All Rh-negative patients should receive at least one full dose of RhIG at 28 weeks gestation and another full dose after birth (assuming that the newborn is found to be D-positive or weak D-positive).
- Canada: Society of Obstetricians and Gynaecologists of Canada (2018): Antepartum RhIg dose of 300 µg (1500 IU) at 28 weeks (alternatively, 2 doses of 100–120 µg, one at 28 weeks and one at 34 weeks). After delivery of a D-positive infant, another 300 µg (alternatively, 120 µg IM or IV) within 72 hours of delivery.
If it is determined that the FMH was greater than 30 mL of whole blood, additional doses must be administered. Administration of both an antepartum and postpartum dose of RhIG has significantly reduced the risk for the production of anti-D in Rh-negative patients who deliver Rh-positive babies. The current rate of RhIG "failure" is approximately 0.1%.
Note: Because RhIG contains IgG anti-D when given antepartum, it can cross the placenta and sensitize fetal D-positive red cells. Occasionally, the fetus may be born with a weakly positive DAT, but significant hemolysis does not occur. For this reason, some guidelines recommend that labs do NOT routinely perform DATs on infants whose mothers have received antepartum RhIG.
