Fetomaternal hemorrhage (FMH) greater than 30 mL of whole blood occurs in only about 0.3% of cases but must be detected to prevent the mother from producing anti-D. Once the mother has become immunized to D, RhIG is of no use.
A typical test protocol is to first screen for a large FMH and then quantify the bleed if the screen is positive. Some laboratories proceed directly to a test that can quantitate the size of the FMH.
Once the size of the FMH is determined, a formula is used to determine how much RhIG is needed. Recall that:
- A standard vial of RhIG contains 1500 IU (300 µg) of IgG anti-D.
- 300 µg of RhIG can suppress immunization to approximately 30 mL of D-positive whole blood.
