Objective To observe the pharmacokinetics of intramuscular anti-D immunoglobulin (IgG) given for routine antenatal prophylaxis.
Design Prospective observational study.
Setting Maternity unit and antenatal serology laboratory in a district teaching hospital.
Population Forty-five rhesus-D-negative pregnant women not sensitised to RhD.
Methods Serial serum quantitations of anti-D IgG following the intramuscular injections of anti-D IgG 100 μg (500 IU) at 28 and 34 weeks of gestation. Anti-D IgG concentrations were assayed with the RFA-300 continuous flow analyser.
Main outcome measures The kinetic profile and duration of detectable anti-D IgG in maternal serum following the first and second injections of anti-D IgG.
Results For the 43 women in whom serial data were collected, there were no detectable differences between pregnancies with an RhD-positive (26) or -negative (17) fetus. Maximum IgG concentrations were detected two to five days following the first anti-D IgG injection and ranged between 0 and 28 ng/mL. Only 30% of women still undelivered at 40 weeks of gestation had detectable IgG at 2 ng/mL or greater. There was a significant relationship between higher maximum values and low maternal surface body area (R2= 0.204, P= 0.002), but this did not influence duration of persistent IgG.
Conclusion Using previously published data, 70% women are not adequately protected with anti-D IgG 12 weeks after the first prophylactic injection. Despite this, previous clinical results suggest that the antenatal prophylaxis schedule used provides adequate protection and that the recommendation for the lowest concentration of protective anti-D IgG antibody levels currently in use is probably overestimated.