Unfortunately, there is no definitive test to determine if anti-D is passive or active. Tests can be done that will suggest whether the anti-D is passive or active; however, many laboratories do not perform such testing routinely. It's both safe and efficient to consider a post-RhIg anti-D to be passive without further testing, thereby triggering a post-natal RhIg injection. Further testing would be done if serologic test results suggest an immune anti-D (eg, 4+ reactions with D+ red cells).
Some TS laboratories try to determine if anti-D is passive or
immune by performing titrations to determine the titer of the anti-D. Such a
protocol usually assumes that an anti-D titer greater than 4 likely represents
active immunization. Unfortunately, a titer of 4 or 8 could be active or passive,
although a high titer (e.g., 64 or more) almost certainly means the anti-D is
Titration results can be affected by several variables:
antigen variability (even if the same phenotype);
Because lower titers could be due to both passive and immune
anti-D, in the absence of test results
that suggest immune anti-D, routine antibody titration is not a
good use of time compared to assuming that anti-D is passive.
Most transfusion medicine best practice guidelines do NOT
routine titration for women known to be injected with RhIg and exhibiting a 2+
or less reaction with D+ red cells, i.e., test results consistent with
RhIg-derived passive anti-D.