Abstract
A sensitive test for the presence of D-positive fetal red blood cells (RBCs) in the maternal circulation of D-negative women has been developed. It was used to investigate the possibility that the occasional failure in preventing alloimmunization might be due to the administration of inadequate amounts of prophylactic anti-D Rh immune globulin. The standard dose in Australia contains 125μg of antibody, and can suppress immunization by an estimated 6 mL of packed D-positive RBCs. A fetomaternal hemorrhage (FMH) of this volume is detectable in the maternal circulation as approximately 0.25 percent of the total RBCs. Our test utilizes a commercially available human monoclonal IgG anti-D that has been biotinylated and used with a dye-conjugated streptavidin. Flow cytometry is used to quantitate fluorescing D-positive RBCs. To date, 2,288 tests have been performed on blood samples from D-negative women attending local antenatal clinics or at the time of delivery. Evidence for an FMH has been obtained in six cases (0.26%). In one case, the FMH was only 0.1 percent, and in another (confirmed by the Kleihauer-Betke method), fetal cells constituted only 0.2 percent. Additional Rh immune globulin was not given to these patients. In the other four cases, the D-positive fetal cells were estimated to be 0.7, 0.5, 0.5, and 0.4 percent, and additional prophylactic Rh immune globulin was administered. Although the prevalence of FMH is low, screening D-negative women at risk of alloimmunization has proved to be simple, fast, and inexpensive.