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Hemolytic disease of the newborn (HDN) is caused by the premature hemolysis of fetal erythrocytes by alloantibodies of maternal origin. These antibodies are produced by previous pregnancy or transfusion. When fetal RBCs cross the placenta and enter the maternal circulation they may stimulate the production of maternal antibodies against any fetal RBCs. Antibodies of the IgG class can cross the placenta into the fetal circulation, bind to fetal RBCs and cause their immune destruction. Only one to two percent of women form alloantibodies during pregnancy. Anti-A, -B, and -D cause 88 to 97% of the cases of HDN. Hemolysis due to other antibodies is rare. ABO and Rh typing and a direct antiglobulin test are performed on cord bloods of infants born of group O or Rh negative mothers, since they are at highest risk of developing HDN. Cord blood samples from all other mothers are saved for seven days in case signs and symptoms of HDN develop. ABO blood grouping is performed on cord bloods to determine if fetal-maternal ABO incompatibility exists. Fetal-maternal ABO incompatibility causes two-thirds of all cases of HDN. Most cases of ABO HDN occur in A or B infants of group O mothers, since group O mothers produce higher quantities of IgG anti-A and -B. The anti-A or -B present in group B or group A mothers is usually composed of IgM and does not cross the placenta. ABO HDN cannot occur if the father is group O, the mother is group AB, or the infant is group O. Rh incompatibility causes fewer cases of HDN, but they are usually more severe. Rh typing has two purposes: (1) to determine if fetal-maternal incompatibility exists, and (2) to determine if the Rh negative mother is a candidate for Rh immune globulin. A direct antiglobulin test of the infant's RBCs is necessary to diagnose or exclude HDN. It determines whether or not the infant's RBCs are coated with antibody. A positive direct antiglobulin test confirms that the infant's RBCs are coated with antibody and supports the diagnosis of HDN. The strength of the direct antiglobulin test does not correlate with the severity of disease. In many cases of ABO HDN, a positive direct antiglobulin test is more often found on cord blood samples than on newborn samples drawn after the baby becomes symptomatic. With the exception of ABO HDN, which may have very weak or negative direct antiglobulin tests, a negative test suggests that anemia and jaundice are due to an nonimmune etiology. Specimen requirement is one 10 mL plain red top tube collected with needle and syringe to avoid contamination with maternal blood and debris. Tube must be labeled with mother's full name and birthdate. Both the requisition and tube should indicate the specimen is cord blood. |
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