Rhogam is often administered to D-negative women during pregnancy to reduce the chances of becoming alloimmunized. The level of antibody can persist for several weeks and may be rarely detected up to 6 months after administration.
Factors affecting duration of effect:
(1) total dose administered
(2) bleeding or increased rate of loss
(3) number of D-positive cells to bind to
Assuming first order kinetics (a simplification):
concentration at time T in ng/mL =
= (maximum concentration in ng/mL) * EXP((-1) * 0.693 / (half life in days) * (T days since administration))
percent remaining =
= 100% * EXP((-1) * 0.693 / (half life in days) * (T days since administration))
Values from Package Insert |
after 300 µg (1 vial) |
after 1,200 µg (4 vials) |
maximum concentration |
37.1 ng/mL |
146.3 ng/mL |
half-life in days |
24.2 |
27.0 |
volume of distribution |
8.59 liters |
8.16 liters |
where:
• The kinetic data is based on 6 patients with 1 dose and 2 patients with 4 doses.
• The route of administration was intramuscular (IM).
• The peak concentration on 4 doses was 36.5 ng/mL per dose.
Based on this data, I have used the following data for half-life in the implementation, which has not been validated.
Number of Vials |
half-life in days |
1 |
24.2 |
2 |
25 |
3 |
26 |
4 |
27 |
According to the AABB Technical Manual, the half life of Rhogam is 21 days in the absence of significant fetomaternal hemorrhage. If a singled 300 µg dose is administered at 28 weeks of gestation, then 7-10% (20-30 µg) should remain at 40 weeks of gestation.
Specialty: Obstetrics & Gynecology, Clinical Laboratory
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