Here's a genetic test I may consider recommending

A mother with Rh- blood who gives birth to an Rh+ infant may potentially develop antibodies to the Rh factor, and the resulting antibodies could cause considerable risk to the next developing fetus. To counteract such a situation, mothers who are Rh- and have an Rh+ mate are traditionally treated at delivery to prevent the development of the Rh antibodies.

The report of an article (Finning K et al. BMJ, 2008;10:1136) in the British Medical Journal describes the study of a new, non-invasive genetic test for the Rh status of a developing fetus. It uses "cell-free fetal DNA present in the maternal blood." The test, according to the report, had a 95.7% success rate when results were compared with the actual blood group tested at birth. False negatives (identifying an Rh+ fetus as Rh-) were "below 0.2% (3 out of approaching 1900 cases)". In these situations, the mother should have been treated but was not. If these mothers were exposed to Rh+ cells from the baby at birth and they became pregnant with another Rh+ fetus, their antibodies might cause a serious hemolytic disease, destroying blood cells in the fetus at or before birth.

From the 95.7% success rate, we can assume that false positives (identifying an Rh- fetus as Rh+) are more common. However, these women would receive the currently standard treatment at birth. So, no additional risk is involved. The current treatment--anti-RhD immunoglobulin (antibodies, effector molecules of immune responses) to effectively block any RhD-positive fetal cells that enter the maternal circulation and prevent the maternal immune system from recognizing them--is a highly effective in preventing hemolytic disease of the newborn.

Since the technique uses "fetal genetic material from the maternal bloodstream" and "is suitable for high-throughput analysis", the authors propose that it is a feasible approach to prevent unnecessary anti-RhD administration for almost all RhD-negative mothers." Before the test can be available as a screening test, "the cost-efficacy of the technique when compared with blanket administration of immunoglobulin . . . must be considered.

Two caveats:

  • The test (and the article) are based on work done in the United Kingdom, not the United States.
  • Companies in the US and elsewhere, selling "fetal testing based on maternal blood" kits have been sued because of inaccurate results.

Yes, I might consider recommending this test. Any immune-system intervention that can be avoided if unnecessary has a lot going for it.

Marie L. Godfrey, PhD


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