Laboratory findings:
(1) The mother is D-negative (dd), with Du taken as D-positive.
(2) The fetus is D-positive.
(3) The father of the fetus is D-positive.
(4) The mother has anti-D in her plasma, which is usually a mixture of IgM and IgG antibodies.
(5) The titer of the anti-D rises during a pregnancy if the mother has been previously sensitized to D-antigen.
(6) The presence of large amounts of anti-D coating the fetal red blood cells may interfere with D antigen typing. In this case the direct antiglobulin test will be positive and the D antigen can be detected if the antibody is eluted from the red blood cells.
(7) An eluate prepared from fetal red blood cells will show anti-D.
(8) The autocontrol for antibody testing in the mother will be negative.
Differential diagnosis of a positive DAT (direct antiglobulin test) in cord blood:
(1) ABO-incompatibility between mother and fetus
(2) anti-D from Rhogam administration
(3) other alloantibody
(4) autoantibody
Anti-D associated with Rhogam administration:
(1) There is a history of Rhogam administration (although this may not always be recalled by the patient).
(2) The antibody is pure IgG.
(3) The antibody titer falls rather than rises over time.
(4) After 3 months the mother;s plasma should not have anti-D activity.
(5) It can be eluted from the fetal red blood cells.
