Study Supports New Test To Target Pregnancy Blood Disorder

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Main Category: Pregnancy / Obstetrics
Also Included In: Genetics;  Blood / Hematology
Article Date: 04 Apr 2008 - 0:00 PDT

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According to a study published on bmj.com, researchers have developed a new test that can identify whether a mother and her baby have different blood types. Kristin Finning (International Blood Group Reference Laboratory, NHS) and colleagues maintain that the test is accurate, feasible, and could significantly reduce unnecessary treatment.

If a mother's blood type is Rhesus (Rh) negative and she is carrying a baby whose blood is Rh positive, complications can occur. Blood that is Rh positive contains an RhD antigen that passes into the mother's blood during birth. This substance causes the mother to make antibodies against the RhD positive blood.

During a woman's first pregnancy, there are usually no problems due to the antibodies. However, if the woman has another RhD positive baby, the antibodies can cross the placenta and invade the baby's red blood cells. This results in hemolytic disease - a blood disorder that is serious and can cause death.

The preventive measure for this disorder is to test the blood type of pregnant women at their first antenatal visit. If the woman is RhD negative, she is given one or two antiserum injections of anti-RhD immunoglobulin during the pregnancy. Inefficiencies arise in this system, however, because about 38% of RhD negative women are carrying a baby who is also RhD; these women receive an unnecessary treatment of antiserum injections.

To avoid the occurrence of unnecessary treatments, researchers at the NHS Blood and Transplant Centre in Bristol investigated a new way to predict a baby's blood group by "typing" its DNA in the plasma of RhD negative pregnant women.

The researchers took blood samples from 1,997 women at or before the 28 week doctor visit and found that the correct RhD phenotype of the baby was predicted by genotyping tests in 96% of cases. Confirmation of this was made by testing blood samples retrieved from the umbilical cord at delivery. Fourteen blood samples (0.8%) resulted in false positive outcomes and three blood samples (0.2%) were false negatives. Unobtainable or inconclusive results occurred in 3.4% of cases.

The researchers point out that if the results of the test had been used as a guide for whether or not a woman would receive the antiserum treatment, only 2% of women would have received anti-RhD without necessity - compared to 36% who unnecessarily receive the antigen without genotyping.

"Our results show that fetuses of RhD negative women could be RHD genotyped with an acceptable level of accuracy," write the authors. They also point out the low rate of false positive results. "The introduction of fetal genotyping followed by the withholding of antenatal anti-RhD prophylaxis from mothers with an RhD negative fetus would result in about 36% of women being saved from unnecessary exposure to human blood products, inconvenience, and discomfort."

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
Kirstin Finning, Pete Martin, Joanna Summers, Edwin Massey, Geoff Poole, Geoff Daniels
BMJ. (2008).
doi:10.1136/bmj.39518.463206.25
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Written by: Peter M Crosta
Copyright: Medical News Today
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