What cells will be agglutinated by the eluate of a newborn with a positive Direct Antiglobulin Test (DAT) with anti-Immunoglobulin G (IgG), whose mother is O, Rh-negative and received RhoGAM (Rh Immune Globulin) at 28 weeks' gestation?

Analysis of Positive DAT in Newborn with Rh-Negative Mother

The most likely result of the eluate experiment is option C, where only the O, Rh-positive cells will be agglutinated by the child's eluate at AHG phase.

Background and Clinical Scenario Analysis

This case involves a 19-year-old O, Rh-negative woman who delivered her first child, who is A, Rh-positive. The mother received RhIg at 28 weeks' gestation, has a negative indirect antibody screen, and has never been transfused. The child has a positive DAT (2+ with anti-IgG), but a negative indirect antibody screen.

Understanding the Positive DAT in this Case

The positive DAT in the newborn indicates the presence of antibodies coating the baby's red blood cells. Given the clinical scenario, there are several potential explanations:

1. Passive transfer of anti-D from maternal RhIg administration:

   • The mother received RhIg at 28 weeks' gestation
   • RhIg contains anti-D antibodies that can cross the placenta
   • These antibodies can coat the D-positive red cells of the fetus

2. ABO incompatibility:

   • Mother is blood group O and baby is blood group A
   • However, the eluate experiment results rule this out

Analysis of Eluate Experiment Results

The eluate experiment tests which donor cells will be agglutinated by antibodies eluted from the baby's red cells:

• Option C shows agglutination only with O, Rh-positive cells
• This pattern indicates that the antibody in the eluate is specific for the Rh(D) antigen
• The antibody does not react with A, Rh-negative or B, Rh-negative cells, ruling out anti-A or anti-B

Explanation of the Findings

The positive DAT in this case is due to passive transfer of anti-D from the RhIg administered to the mother at 28 weeks' gestation 1. The anti-D antibodies crossed the placenta and coated the D-positive red cells of the fetus, resulting in a positive DAT.

Key points supporting this conclusion:

• The mother is Rh-negative and received RhIg during pregnancy
• The baby is Rh-positive, providing a target for the anti-D antibodies
• The eluate reacts only with Rh-positive cells, confirming anti-D specificity
• The mother's negative antibody screen indicates she has not actively produced anti-D antibodies

Clinical Implications

This finding is consistent with passive transfer of anti-D from RhIg rather than maternal alloimmunization. While RhIg-mediated hemolytic disease of the newborn is rare, it can occur 1. In this case:

• The baby may be at risk for mild hemolysis due to the anti-D coating the red cells
• Monitoring for jaundice and anemia would be appropriate
• The condition is typically self-limiting as the passively transferred antibodies gradually disappear

Important Considerations

1. Distinguishing from true maternal alloimmunization:

   • In true alloimmunization, the mother would likely have a positive antibody screen
   • The negative maternal antibody screen supports passive transfer from RhIg

2. Utility of eluate testing:

   • Eluate testing is valuable in determining the specificity of antibodies coating the red cells 2
   • In this case, it clearly demonstrates anti-D specificity

3. Prevention of Rh alloimmunization:

   • RhIg administration at 28 weeks and after delivery is standard practice for Rh-negative mothers 3
   • This prevents maternal sensitization to the D antigen, protecting future pregnancies

The pattern of reactivity seen in option C (agglutination only with O, Rh-positive cells) is diagnostic of anti-D antibodies coating the baby's red cells, consistent with passive transfer from maternal RhIg administration.