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MS WordPerinatologist Corner - C.E.U/C.M.E. Modules
Antibody Screen Positive: Rh Disease and Other Atypical Antibodies
Sponsored by The Indian Health Service Clinical Support Center
5. Rh Disease and Other Atypical Antibodies
Step 5. Screening
Clinical Scenario
….Let’s get back to the patient we mentioned before.
How do you work up the woman whose prenatal lab work returns with a positive antibody screen?
What does the test mean?
The work-up and management of the woman with Rh sensitization, or sensitization to any of the other atypical antibodies, is a bit complicated, but appropriate for the primary care provider. The work-up needs to be taken step by step, with referral when a certain stage has been reached. The first thing to verify when a woman’s prenatal antibody screen test (“AST”) returns “positive” is to verify the type of antibody present and its titer. Most, but not all, labs will automatically provide this information. The “AST” is more correctly known as an indirect Coombs test, which detects circulating antibody in maternal serum. Most of these are 7S gamma G type antibodies which can attach to their respective antigen proteins on the red cell surface. Bound antibody is detected with an anti-gamma G antibody, which is mixed with the infant’s red cells, and is known as a direct Coombs test. This is the correct test to order in the jaundiced newborn. As you will recall, the titer is the tube dilution at which antibody is still detected in that individual. Different labs will have different titers which are considered significant, but most will consider 1:8 or 1:16 titers significant. The higher the titer, the greater amount of antibody that is present. A rising titer over time means that the maternal anamnestic response is being activated, especially if the titer rises more than a four-fold tube dilution (see below).
About 1% of prenatal patients will be found to have a positive antibody screen. Anti-D, which is the antibody directed against the Rh antigen, is the most common, but a variety of other antibodies may be found. Antibodies may be directed at other antigens at the Rh locus: C, c, E, and e, and are relatively common. Others that commonly may be associated with hemolytic disease are Kell (K), Duffy (Fya), and Kidd (Jka). All these antibodies are of the IgG class and can cross to the fetus.
Antibodies directed against the Lewis a and b (Lea, Leb) antigens, and anti-P antigens, are of the IgM class and do not cross the placenta, and so are not of perinatal consequence. “Warm” or “cold” antibodies likewise do not have perinatal importance, but women who have such antibodies, and who may need a transfusion, will require that the transfused blood be warmed prior to being administered.
