The dosage of anti-D immune globulin is calculated from the estimated volume of the fetal-to-maternal hemorrhage, as described on page 307. One 300-�g dose is given for each 15 mL of fetal red cells or 30 mL of fetal whole blood to be neutralized. If using an intramuscular preparation of anti-D immune globulin, no more than five doses may be given in a 24-hour period. If using an intravenous preparation, two ampules-totaling 600 �g-may be given every 8 hours. To determine if the administered dose was adequate, the indirect Coombs test may be performed. A positive result indicates that there is excess anti-D immunoglobulin in maternal serum, thus demonstrating that the dose was suicient. Alternatively, a rosette test may be performed to assess whether circulating fetal cells remain.
Formerly called D") these are the most common antigenic D variants in the United States and Europe. Serological weak D phenotypes have been further refined into two general categories using molecular analysis-RHD genotyping. Molecular weak D phenotypes carry reduced numbers of intact D antigens on the red cell surface. Those designated partial D types have protein deletions associated with abnormal D antigens that lack epitopes (Sandler, 2017). When this distinction is known, it can have clinical consequences in terms of sensitization risk and need for anti-D immune globulin.
Traditionally, serological weak D individuals have been considered to be D-positive or -negative depending on the clinical situation. For the purposes of blood donation, they are categorized as D-positive, whereas transfusion recipients with weak D are considered D-negative. In pregnancy, weak D has also been considered D-negative, so that individuals receive immune globulin and avoid potential sensitization (American College of Obstetricians and Gynecologists, 2017; Sandler, 2015).
Many non-Hispanic white Americans who test positive for weak D have weak D phenotypes 1, 2, or 3. Individuals with these phenotypes may be managed as though they are D-positive. Because they are not at risk for alloimmunization, anti-D immune globulin is not needed (Sandler 2015, 2017). In contrast, individuals with partial D antigens may be at risk for D-sensitization and do require immune globulin. Molecular RHD genotyping has been suggested for pregnant women with weak D phenotype, bur cost-benefit analysis of this strategy is presently lacking (American College of Obstetricians and Gynecologists, 2017). f molecular genetic testing has not been peormed in those with serologic weak D phenoype) D immunoprophylaxis should be administered to those with weak D phenotype.
A small amount of fetomaternal bleeding likely occurs in all pregnancies, and in two thirds, this may be suicient to provoke an antigen-antibody reaction. As shown in Figure 15-2, the incidence increases with advancing gestation and the 80 0.19 mL 70 )> 60;;)>C 50)C·uE 40