A 25-year-old woman, gravida 2 para 1, at 8 weeks gestation comes to the physician to initiate prenatal care. Her blood type is O negative and the father's blood type is O positive. Her first pregnancy was significant for placental abruption at the time of delivery. She received the standard dose of anti-D immune globulin at 28 weeks during her first pregnancy and again 1 day postpartum. The patient has no medical problems and no history of blood transfusions. Her anti-D antibody titer is currently 1:32. Which of the following is the most likely explanation for this patient's finding?

Question

A 25-year-old woman, gravida 2 para 1, at 8 weeks gestation comes to the physician to initiate prenatal care.  Her blood type is O negative and the father's blood type is O positive.  Her first pregnancy was significant for placental abruption at the time of delivery.  She received the standard dose of anti-D immune globulin at 28 weeks during her first pregnancy and again 1 day postpartum.  The patient has no medical problems and no history of blood transfusions.  Her anti-D antibody titer is currently 1:32.  Which of the following is the most likely explanation for this patient's finding?

Quizplus · Accepted Answer

This woman's O-negative blood type indicates that she is Rh negative, and the anti-D antibody titer of 1:32 reflects that she is alloimmunized (ie, sensitized). Alloimmunization occurs when the mother is Rh negative and has an Rh-positive fetus. Her first pregnancy, especially the placental abruption, put her at risk for alloimmunization. To prevent the maternal immune system from developing anti-D antibodies, anti-D immune globulin is first administered at 28 weeks gestation and repeated within 72 hours of delivery. Alloimmunization A standard dose of 300 µg at 28 weeks gestation can usually prevent alloimmunization. However, ~50% of Rh-negative women will need a higher dose after delivery, placental abruption, or procedures. The Kleihauer-Betke (KB) test is commonly used to determine the dose. Red blood cells from the maternal circulation are fixed on a slide. The slide is exposed to an acidic solution and adult hemoglobin lyses, leaving "ghost" cells. The dose of anti-D immune globulin is calculated from the percentage of remaining fetal hemoglobin. Kleihauer-Betke (KB) test (Choice B) The standard dose of anti-D immune globulin at 28 weeks gestation in an uncomplicated pregnancy is usually adequate as the risk of alloimmunization is very low before this time. However, if the patient developed placental abruption earlier in pregnancy, the KB test should be performed to determine whether a higher dose is indicated. Other indications for anti-D immune globulin include amniocentesis, chorionic villus sampling, and external cephalic version. (Choice C) Prophylaxis between pregnancies is rarely necessary. It is possible but uncommon for sensitization to occur from inadvertent intravenous transfusion of Rh-positive blood due to routine blood bank cross-matching. Sensitization could also occur during an unrecognized spontaneous abortion of an Rh-positive fetus, but most women would experience bleeding. (Choice D) Delayed administration of anti-D immune globulin after 72 hours postpartum may allow sensitization to occur. However, the timing of this patient's anti-D immune globulin within 72 hours after delivery is appropriate. (Choice E) The patient's antibody titer reflects Rh alloimmunization that developed from the first pregnancy. If an undisclosed new partner is Rh positive, the fetus may also be Rh positive and at risk for hemolytic disease of the newborn. However, if an undisclosed new partner is Rh negative, the fetus will not be at risk for hemolysis. Educational objective: Anti-D immune globulin should be administered to any Rh D-negative mother who delivers an Rh D-positive baby. The standard dose is usually adequate at 28 weeks gestation. After delivery or procedures, the Kleihauer-Betke test is used to determine whether a higher dose is needed due to the increased risk of fetal blood cells entering the maternal circulation. References: ACOG practice bulletin - prevention of Rh D alloimmunization. (http://www.ncbi.nlm.nih.gov/pubmed/10458556)

A 25-Year-Old Woman, Gravida 2 Para 1, at 8 Weeks