Numerous studies have shown that, if administered correctly, RhIG is effective at preventing D immunization. To work, RhIG must be given in a sufficient dose, and it must be given before Rh immunization has begun.
Unfortunately, despite RhIG's proven efficacy, some patients continue to make anti-D in the perinatal period. Such 'failures' are mainly (but not totally) due to human error.
Examples of how patients may still produce anti-D some 40+ years after the implementation of RhIg prophylaxis:
- Immunization to D occurred before the administration of RhIG, e.g., before 28 weeks gestation.*
- Immunization to D occurred after the administration of RhIG at 28 weeks and before delivery, because an antenatal fetomaternal hemorrhage (FMH) occurred that was too large for residual passive anti-D to give protection.
- Patient was already immunized from a prior pregnancy but anti-D was too weak to be detected in antibody screen tests prior to RhIG administration.
- RhIG dosage was insufficient to clear a larger fetal bleed at delivery, e.g., FMH screen was not done or a false negative occurred.
- Incorrect calculation of RhIG dosage.
- RhIG administered too late, i.e., well after 72 hours of delivery.
- Antenatal RhIG not given, e.g., mother had no, or limited, access to prenatal care or did not seek it, and an FMH occurred during pregnancy.
- Failure of a physician to carry out prenatal blood testing.
- RhIG not given due to laboratory clerical or technical error in Rh typing the mother or child.
- RhIG not given in cases such as abortions, ectopic pregnancies, and trauma (e.g., car accidents).
*Because anti-D production before 28 weeks is rare (0.24% to 0.31%), RhIG's use earlier in pregnancy is not recommended. It is not cost-effective and would expose most patients to an unneeded blood product.
