What is a fetal-maternal hemorrhage (FMH)?
A fetal-maternal hemorrhage is a bleed across the placenta from fetus to mother before or during delivery. Other than during normal delivery, this can result due to a trauma, amniocentesis, fetal blood sampling, miscarriage, or abortion.
Why do we need to know if this has occurred?
We need to establish if a hemorrhage has occurred in RhD negative mothers who have RhD positive babies to help prevent the mothers from becoming sensitized to the D antigen which is present on the red blood cells of RhD positive babies.
What is given to RhD negative mothers to prevent this sensitization?
Rh immune globulin or RhoGAM is given to RhD negative mothers to prevent sensitization.
When is RhoGAM given?
One dose of RhoGAM is given at 28 weeks of gestation and also again within 72 hours of delivery of an RhD positive baby.
One dose of RhoGAM can protect against how many milliliters of RhD positive blood?
One dose of RhoGAM can protect against 30 mL or less of RhD positive blood.
What is RhoGAM?
RhoGAM is an Rh(D) immune globulin of mostly IgG anti-D from human plasma made by Ortho-Clinical Diagnostics and has been around since 1968. Prior to 1968, hemolytic disease of the fetus and newborn (HDFN) was a major cause of infant mortality in the U.S., but the incidence of HDFN currently, for the most part, has been eliminated. The dosing frequency of RhoGAM brand is 12 weeks, so patients getting RhoGAM at the 28 weeks should be covered through the normal 40-week gestation period. If RhoGAM is given before 28 weeks of gestation, it is advisable to repeat the dose every 12 weeks to ensure adequate levels until delivery.
What is HDFN?
HDFN stands for hemolytic disease of the fetus and newborn, and the condition is also known as erythroblastosis fetalis. HDFN occurs when the mother is exposed to antigens that the baby’s red blood cells possess, but her red blood cells lack. Such antigens are inherited from the father. The mother mounts an immune response to these antigens which are considered foreign to her by making antibodies against them. The antibodies cross the placenta and coat the baby’s red blood cells, accelerating the destruction of the baby’s red blood cells, which causes anemia, jaundice, and possibly heart failure. HDFN is often classified into three categories with anti-D being the most severe causative antibody, followed by antibodies against other antigens in the Rh system or other systems, and least severe are antibodies of the ABO system. In this module, we are focusing on the scenario where the baby possesses the D antigen, but the mother lacks the D antigen. So, when the mother is exposed to a large volume of baby D antigen positive cells, she may produce anti-D if not given RhoGAM to prevent this. Rh sensitization is usually not likely in the first pregnancy, but will affect future pregnancies if the baby is RhD positive or possesses the D antigen.
What is the principle of the Fetal Screen II procedure?
In the Fetal Screen II procedure, a chemically modified anti-D (antibody reagent) is mixed with a 2–3% red blood cell suspension of the mother and incubated for 15 minutes at 37 degrees C. If the baby’s RhD positive cells are present in the mother’s cell suspension, one part of the Fab piece of the modified anti-D molecule will attach to the D antigen of the RhD positive cells. After incubation, the mixture is washed four times with saline, decanting after each wash, to remove unbound antibody (antibody reagent). After the final wash, one drop of RhD positive indicator cells are added. These indicator cells, which have the D antigen, will attach to the open second part of the Fab piece of the modified anti-D molecule. The result is the appearance of rosettes or mixed-field agglutinates. The more baby RhD positive cells present, the more rosettes will be seen. If there are 3 or more rosettes seen using low power magnification in a total of 5 fields, then that is an indication that a large fetal-maternal hemorrhage (greater than 30mL of whole blood) has occurred. A quantitative test such as the Kleihauer-Betke acid elution procedure or the use of flow cytometry must then be done to determine if additional dosages of RhoGAM are necessary.
What are some limitations of the procedure?
If an infant is tested as being weak RhD positive, a test based on fetal hemoglobin is recommended because weak RhD positive red blood cells have fewer D antigen sites and may not react as well as RhD positive red blood cells. If the mom has a positive direct antiglobulin test, this procedure may produce a false positive test result. Use this test only for RhD negative mothers. This test can only be used to detect FMH in cases of RhD incompatibility.
Excessive washing may result in false negative results. Insufficient washing may result in false positive results. If a positive result is obtained in this procedure, do not assume that additional dosages of RhoGAM are indicated. Further testing is required. The presence of ABO incompatibility between mother and fetus may result in a false-negative test result because of the accelerated rate at which the incompatible fetal cells are being cleared from the maternal circulation. After the one drop of Fetal Screen II indicator cells are added to each washed tube and mixed well, the tubes are centrifuged for 15–45 seconds at 3400 rpm. The size, number, and quality of rosettes may be improved if the spin time is extended to 45 seconds.
What is the purpose of the Fetal Screen II test procedure? The procedure is done to determine if there are RhD positive fetal red blood cells present in the circulation of an RhD negative mother that could have occurred from a fetal-maternal hemorrhage. Is this a qualitative or quantitative test? It is a qualitative test. So, what will you do in this simulation? You will prepare the appropriate cell suspension of the mother’s red blood cells and test it to determine if there are any of the baby’s red blood cells present in it by looking for rosettes or mixed-field agglutinates microscopically.
