An issue related to reaction strength of RhIg in serologic tests is how long passive anti-D from RhIg can be detected post-injection.
The half-life of IgG is 23 to 26 days. Following injection of RhIg, serologically detectable levels of anti-D peak within hours (IV injection) or days (IM injection).
Although the half-life of passive anti-D from RhIg is approximately 3 weeks, it may be detectable by serologic tests for approximately 8 weeks by the indirect antiglobulin test (IAT) and up to 12 weeks or more by continuous flow analyzers used to quantify anti-D. Levels of passive anti-D will decrease over time.
Immune anti-D becomes detectable later (eg, about 4 weeks after exposure to D+ red cells), and generally reaches a peak after 6–8 weeks. Levels of immune anti-D will remain constant for longer and will increase following exposure to another immunizing dose of fetal D+ cells.
Depending on the many variables that can affect reaction strength (mentioned earlier), as detected serologically, passive anti-D from RhIg can be detected for about 8 weeks or longer by routine, sensitive antibody detection methods.
Since RhIg is injected at about 28 weeks, it is routinely detected at delivery, which could occur well before the ~40 weeks considered to be normal gestation (37–42 weeks by the World Health Organization).