Indirect Coombs test in Pregnancy
المؤلف:
Marcello Ciaccio
المصدر:
Clinical and Laboratory Medicine Textbook 2021
الجزء والصفحة:
p423-424
2025-11-25
24
The Coombs test evaluates a possible incompatibility between maternal and fetal blood through the detection of maternal immunoglobulin (Ig)G antibodies that, crossing the placenta, can develop an immunological response against antigens of fetal erythrocytes, leading to severe consequences for the fetus, such as intrauterine death and hemolytic dis ease of the newborn (HDN), characterized by anemia, neo natal jaundice, edema, and hydrops. The antibodies involved may be different (anti-Kell, anti-C, anti-E, etc.), but in most cases, they are anti-D. In particular, the Coombs test is used to evaluate the possible presence of maternal antibodies directed mainly against the erythrocyte Rh antigen, also known as the D antigen.
The Rh factor is a protein present on the surface of red blood cells in about 15% of the population that is, therefore, defined as Rh+; the remaining 85% of the population does not present this antigen on red blood cells and is, therefore, defined as Rh−. Unlike the AB0 system, Rh subjects do not present anti-Rh antibodies in the serum unless exposed to Rh+ blood cells (e.g., transfusion). The Rh factor is inherited as an autosomal dominant trait, so if the mother is Rh− and the father is Rh+, the fetus will likely be Rh+ and, therefore, there may be an incompatibility between maternal and fetal blood. Under normal conditions, the placenta acts as a barrier between maternal and fetal blood; only small amounts of fetal blood can reach the maternal circulation. However, these are generally insufficient to induce an immune response producing antibodies to Rh+ fetal blood cells. However, special conditions, such as previous pregnancies, blood transfusions, or promiscuous use of syringes, can lead to maternal immunization to D antigen. In addition, invasive diagnostic procedures, such as chorionic villus sampling or amniocentesis, may result in the passage of fetal blood into the maternal circulation. To prevent an Rh− mother of an Rh+ fetus from developing antibodies to the D antigen, she should receive prophylaxis with anti-D immunoglobulin before undergoing invasive testing. At the time of delivery, mainly if traumatic obstetrical maneuvers are performed, or in the case of abortion, large amounts of fetal blood may come into con tact with maternal blood, thus stimulating a process of immunization leading to the synthesis of anti-D antibodies. Therefore, subsequent pregnancies will be at high risk of developing HDN because exposure to the antigen (fetal Rh+ blood cells) triggers a secondary immune response with the production of IgG antibodies that can cross the placenta and damage fetal red blood cells. At birth, the Rh group of the newborn is determined. If the newborn is Rh−, then the mother does not need to undergo further immunoprophylaxis. On the other hand, if the newborn is Rh+ and the mother is Rh−, the mother will have to undergo immunoprophylaxis again.
Thus, Rh− women can get immunized:
• After delivery due to immunoprophylaxis with inadequate or absent anti-D Ig
• During pregnancy due to events associated with transplacental hemorrhages, such as miscarriage, amniocentesis, villocentesis, ectopic pregnancy, intrauterine fetal death, external version maneuvers, or abdominal trauma.
After sensitizing events, postpartum and prenatal anti-D prophylaxis reduces maternal immunization risk. Experimental studies have shown that routine prenatal pro phylaxis in Rh− pregnant women results in an absolute reduction in the risk of immunization. Therefore, all nonsensitized Rh-pregnant women should receive immunoprophylaxis at 28 weeks of gestation.
There is certainly no risk to the fetus if both parents are Rh− or the mother is Rh+.
The Coombs test is performed by drawing maternal blood. Maternal serum/plasma is mixed with Rh+ blood at 37 °C for a sufficient period (45–60 min) for IgG class D antibodies, which may be present in the maternal serum/plasma, to recognize and bind to the D antigen. Coombs serum, an anti- human IgG antiserum, is then added and bridges between the attached antibodies to cause obvious agglutination of the red blood cells. In the presence of agglutination, the test is positive and, therefore, the mother will be Rh+; in the absence of agglutination, the test will be negative and, therefore, the mother will be Rh−.
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