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The page below is a sample from the LabCE course Rh-Negative Female with Anti-D at Delivery: A Case Study. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Passive or Active Immunization?

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 postnatal RhIG injection. Further testing would be done if serologic test results suggest an immune anti-D (eg, 4+ reactions with D+ red cells).
Some transfusion services 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 (eg, 64 or more) almost certainly means the anti-D is immune.
Titration results can be affected by several variables:
  • Red cell phenotype
  • Donor antigen variability (even if the same phenotype)
  • Method used
  • Operator variability
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 recommend routine titration for women known to be injected with RhIG and exhibiting a 2+ or less reaction with D+ red cells, ie, test results consistent with RhIG-derived passive anti-D.