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, it cannot be undone and RhIG is of no use.
A typical test protocol is first to 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.
Several methods are available to detect an FMH that requires additional RhIG. Acceptable screening tests for FMH include:
- Rosette method
- Commercial fetal bleed screening tests
- Gel agglutination fetal cell screening technique
Note: The rosette test may be falsely positive if the mother is weak-D positive and may be falsely negative if the baby is weak-D positive.
