A postpartum screen on an RhIg candidate shows anti-D with titer 8. What is the most likely explanation for this result?

• A. Postpartum transfusion effect
• B. Antepartum administration of RhIg
• C. Laboratory error
• D. Fetal alloimmunization

Answer: (B)

The main idea is that a postpartum positive anti-D screen in someone who received Rh immune globulin (RhIg) is most likely due to the anti-D antibodies that come with the RhIg itself. RhIg is given to RhD-negative mothers to prevent sensitization, and it contains anti-D antibodies. These passively acquired antibodies can be detected by antibody screening tests for weeks after administration, producing a titer such as 8. This finding does not necessarily indicate true maternal alloimmunization to the D antigen; it simply reflects the presence of RhIg-derived anti-D in the circulation.

If this anti-D were from genuine alloimmunization, you’d expect a different pattern—often a rising or persistent antibody titer associated with a history of fetal-metal alloimmune exposure or clinical signs of incompatibility. Transfusion-related or laboratory error could cause false positives, but given the context of RhIg prophylaxis, antepartum administration of RhIg is the most plausible explanation.