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Factors Affecting RhIg Reaction Strength

Red cell reaction strengths at delivery from an antenatal RhIg injection at 26–30 weeks (usually 28 weeks) are typically 2+ or less, although stronger reactions are possible depending on the detection method, time since injection, and other factors.

Multiple variables can affect the reaction strength of passive anti-D seen post-RhIg injection:

  1. Amount of RhIg injected (the greater the number of IU of anti-D administered, the stronger reactions will be);
  2. Titers of anti-D in the plasma pool used to manufacture RhIg (occasionally a donor with an exceptionally strong anti-D may be in the pool);
  3. Maternal physical size and related blood volume (a larger volume of maternal plasma will dilute RhIg more);
  4. Time between RhIg administration and testing (passive antibody will decrease in strength over time);
  5. Sensitivity of antibody detection method (e.g., gel-IAT and PEG-IAT may give stronger reactions than LISS-IAT);
  6. Volume of FMH (amount of D-positive fetal RBC available in the mother to adsorb anti-D);
  7. Route of RhIg administration: Some RhIg products can be administered IM only, whereas others can be given both IM and IV (see later). Peak levels of RhIg are reached faster with IV compared to IM administration (within hours with IV administration compared to days with IM administration). Also, with IV administration, higher levels of IgG anti-D are achieved.
  8. Operator variability (technologist techniques vary in removing cell buttons when reading IATs).

Because of these variables, many laboratories consider 2+ or less reaction strengths to be consistent with passive anti-D.