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MS WordPerinatologist Corner - C.E.U/C.M.E. Modules
Antibody Screen Positive: Rh Disease and Other Atypical Antibodies
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8. Rh Disease and Other Atypical Antibodies
Step 8. Positive antibody screen and titer. What next?
Once you have identified a positive antibody and its titer, what are the next steps?
Once you have identified a positive antibody and know its titer, the next step is to get more history. Has she had a previous infant that had significant jaundice? This may just have been due to physiologic jaundice or ABO incompatibility, but it may raise your suspicion that the antibody in question may again be the cause of a problem. Has she had a previous blood transfusion? This may have been a consequence of a prior postpartum hemorrhage or trauma. Emergency transfusions, especially in years past, often only got a “type and screen” and were not screened for the full panel of possible antibodies because of the urgency of the situation. There remained approximately a 0.3% chance of a subsequent sensitization from one of the rarer antigens. Such transfusion-acquired antibodies are less likely to cause (but do not completely exclude) a potential perinatal problem. For example, approximately 91% of the population is Kell antigen negative, but there is a chance that your patient could have been transfused emergently with the relatively rare Kell antigen positive blood. This could result in sensitization, which could present as a potential problem in a subsequent pregnancy if the anamnestic response were provoked.
Next, test the father of the pregnancy for the antigen against which the antibody is directed. This of course assumes confidence in true paternity and the availability and willingness of the father to be tested. The test you should order for any of the Rh locus genes is a “paternal Rh phenotype”, usually a “send-out” lab. Don’t order an antibody screen on the father; that won’t help! Likewise, his blood type and Rh may or may not help you. The phenotype will tell you if the dad is homozygous or heterozygous for the antigen in question, and thus the risk to the offspring. For other antibodies, order the dad’s phenotype for that antigen (e.g., for a mom with an anti-Kell antibody, order a paternal “Kell phenotype”).
For example, say your patient is Rh negative and has an anti-D antibody detected at a titer of 1:64. Say the dad’s Rh phenotype returns “CDe/cDE”. This means he is a homozygote for “big D” (and a heterozygote for C and E), and implies that the child will have a 100% chance of potentially being affected with hemolytic disease as a result of the maternal anti-D antibody. This pregnancy will certainly require further testing (see below).
As another example, say the mom is Rh positive and has an anti-c antibody at a titer of 1:128. You determine that the paternal genotype is “cDE/cdE”. This dad is homozygous for “little c”, so the child has no chance of being affected by the anti-c antibody: end of problem. (Assuming correct paternity…!) It doesn’t matter what the maternal antibody titer is if the child isn’t going to inherit the antigen against which it is directed.
