55 Prenatal Screening

Principle

The diagnosis and management of hemolytic disease of the newborn (HDN) requires close cooperation between the pregnant patient, her obstetrician, her spouse or partner, and the personnel of the clinical laboratory performing the serologic testing. Serologic and clinical tests performed at appropriate times during the pregnancy can determine the level of antibody in the maternal circulation, the potential of the antibody to cause HDN, and the severity of RBC destruction during gestation. If serologic and clinical data indicate that the fetus is becoming severely anemic, interventions such as intrauterine transfusion can be used to treat the anemia and prevent the development of severe disease.

Specimen

EDTA plasma or serum collected at the first prenatal visit (preferably during the first trimester).

Reagents 

1. Isotonic saline
2. Monoclonal or polyclonal anti-A,-B,-A,B
3. A1 and B red cells
4. Anti-D
5. Rh control reagent
6. Screening cells
7. LISS additive
8. AHG reagent
9. CCs for addition to negative AHG tests

Procedure

1. ABO testing (refer to SOP for ABO)
2. Rh testing (refer to SOP for Rh testing)
3. Weak D testing, if necessary (refer to SOP for weak D testing)
4. LISS antibody screening (refer to SOP for LISS antibody screening)
5. If antibody screening is positive: LISS antibody identification is performed (refer to SOP for LISS antibody screening)
6. Follow up testing depends on the antibody specificity:
   1. Cold reactive IgM antibodies (anti-I, anti-IH, anti-Lea, anti-Leb, and anti-P1) can be ignored and are not usually picked up erroneously if immediate spin and room temperature incubations are not performed as part of the routine antibody screening. Lewis system antibodies are rather common in pregnant women but have not been reported to cause HDN.
   2. Many Rh negative pregnant women have weakly reactive anti-D, particularly during the third trimester as they have received Rh Immune globulin (RhIg) at 28 weeks or because of an event with increased risk of feto-maternal hemorrhage. The presence of this weak anti-D is usually confirmed by reviewing immunization records and performing a “mini” cell panel for weakly positive cells. This weakly reactive, passively acquired anti-D may be demonstrable 2-3 months or longer. This weak anti-D may be present still on delivery, however, if the mother delivers a Rh positive child she will still require RhIg to “top up” her protection.
   3. If the antibody specificity is determined to be clinically significant and the antibody is IgG, further testing is required (other than anti-D the most common significant antibodies are anti-K, anti-E, anti-c anti-C, and anti-Fya).
7. Antibody titers are performed when it has been determined there is a significant antibody present (refer to SOP for antibody titration).

Interpretation

1. Rh-positive Prenatal patients with a negative antibody screen, should have their antibody screen repeated at 28 weeks gestation.
2. Prenatal patients that have a clinically significant antibody should initially have their antibody titers monitored every 4 weeks and increase the frequency if a rapid rise in titer occurs.
3. Rh-negative prenatal patients with a negative antibody screen should have repeat testing at 26 or 27 weeks and 36 weeks. (The 26/27 week screen is so results will be available before 28 week RhIg is required.)

Notes

1. On obstetrical histories, para indicates the number of living children and gravida indicates the total number of pregnancies, including live births, stillbirths, abortions and miscarriages.
2. When prenatal patients are Rh negative, some physicians choose top perform an ABO/Rh test in the dad to see if RhIg
3. RhIg is high titer anti-D. When administered, this passively acquired antibody should remove Rh-positive fetal cells from the maternal circulation before the mother’s immune system can be stimulated to form “true” anti-D.  This product is indicated for Rh-negative women who do not have an immune anti-D. Administration usually occurs at 28 weeks and within 72 hours of delivery if the infant is determined to be Rh positive. The 28 week dose of RhIg  is 300 micrograms and the post-partum dose is usually 120 micrograms(more may be given if at delivery a large number of fetal cells enter the maternal circulation). Fetal maternal hemorrhage amounts are determined at birth, using methods like the Kleihauer test,)which determine the required dose. For more information, consult the Rh Program website at http://rcp.nshealth.ca/rh.
4. Maternal anti-A and anti-B destroy incompatible fetal red cells which enter the maternal circulation, before the fetal cells are able to stimulate the mother to formanti-D so ABO incompatibility may decrease the risk of immunization and the risk of Rh HDN.
5. It is not possible to diagnose ABO HDN prenatally, however, group O women with A, B or AB husbands are most likely to have affected infants (the majority of ABO antibodies are IgM, however most adults have some IgG anti-A and/or anti-B in circulation). The most common form of HDN is due to ABO incompatibility. Generally it is quite mild and the infant does not require treatment.
6. Most severe cases of HDN are due to Rh incompatibility.