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.
