A neonate's D-positive red blood cells may be coated by so much anti-D from the mother that agglutination by anti-D typing reagent may be "blocked".
Features at presentation:
(1) The mother is Rh negative and has anti-D in the serum.
(2) The neonate's serum contains anti-D, which is maternal IgG antibody that has crossed over the placenta.
(3) Typing of the neonate's red blood cells with anti-D is negative (cells do not agglutinate).
(4) The neonate's red blood cells show a very strong reaction in direct antiglobulin testing using anti-IgG reagent.
(5) The neonate is typically anemia, perhaps with hydrops fetalis.
The neonate's red blood cells should be washed in isotonic saline to remove any serum. The cells are then eluted. The eluate should be positive for anti-D.
The cells are eluted until the eluate is negative for anti-D. Retyping the cells with anti-D reagent should identify D antigen.
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