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Test Mom for Fetal DNA and Save an Anti-RhD Ig Shot

Fetal RhD genotyping from maternal plasma would substantially reduce unnecessary administration of anti-RhD immunoglobulin to RhD-negative women with RhD-negative fetuses.

Standard obstetric care for RhD-negative pregnant women includes administering anti-RhD immunoglobulin (Ig) antenatally and after delivery of RhD-positive infants to avoid alloimmunization and potential fetal hemolytic anemia in subsequent pregnancies. The most immunogenic of the Rh antigens, RhD, is encoded by the gene RHD. Tests for predicting fetal RhD status involve amplifying one or more regions of RHD to determine whether and in what form the gene is present. In a study of predominantly white women, approximately 38% of RhD-negative women carried RhD-negative fetuses and thus would receive anti-RhD Ig unnecessarily.

Now, investigators have performed high-throughput RHD genotyping of fetal DNA in plasma from 1997 pregnant women (mean gestational age, 28 weeks) and compared predicted RhD types with cord blood phenotypes. In 96% of cases, the predicted fetal RhD status was the same as that shown in cord blood; the false-positive and false-negative rates were 0.8% and 0.2%, respectively, and the occurrence of inconclusive genotypes was 3%. If this screen had been used on the study cohort to guide practice, only 2% of the women would have received anti-RhD Ig unnecessarily.

Comment: Testing maternal plasma with high-throughput gene amplification could prevent unnecessary administration of anti-RhD Ig to many RhD-negative pregnant women. Because RhD Ig is a pooled human blood product, decreasing its use would save costs and diminish potential risks associated with blood products. In addition, testing would eliminate the need to give anti-RhD Ig for potentially RhD-sensitizing events, such as trauma or miscarriage. If high-throughput maternal plasma testing becomes commercially available, using it routinely would make sense, especially if the savings outweigh the current costs of administering anti-RhD Ig to all RhD-negative women.

— Wendy S. Biggs, MD

Published in Journal Watch Women's Health May 22, 2008

Citation(s):

Finning K et al. Effect of high throughput RHD typing of fetal DNA in maternal plasma on use of anti-RhD immunoglobulin in RhD negative pregnant women: Prospective feasibility study. BMJ 2008 Apr 12; 336:816.

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Kumar S. Universal RHD genotyping in fetuses. BMJ 2008 Apr 12; 336:783.

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• Medline abstract (Free)