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Feasible test for mismatch of mother and baby’s blood

Test could mean over one third of pregnant women don’t have to get treated

Lisa Hitchen

Friday, 04 April 2008

A new test could save over one third of pregnant women from having unnecessary treatment, Bristol researchers have demonstrated.

The team from the International Blood Group Reference Library and NHS Blood and Transplant, Bristol have developed an automated way of working out the phenotype of foetal Rhesus (Rh) D from fetal DNA in the plasma of RhD negative pregnant women.

Current practice is for all pregnant women that are RhD negative to be offered anti RhD immunoglobulin at 28 to 34 weeks gestation, as NICE guidance recommends.

But in a majority white population, only around 38% will be carrying an RhD negative fetus and so are being treated unnecessarily.

Because of this, NICE endorsed studies to look into the feasibility of mass testing of maternal blood to find out infants’ blood groups by analysis of fetal DNA circulating in it.

Such a test would cut use of an expensive blood product in short supply and save patients that didn’t need it the discomfort and risks of exposure to pooled anti-RhD immunoglobulin.

"This risk is exemplified by the infection of hundreds of women with hepatitis C virus transmitted by anti-RhD immunoglobulin in Ireland in 1977-8," the authors wrote in the paper at bmj.com.

The test predicts the baby’s blood group by "typing" its DNA in the RhD negative pregnant woman. Scientists have been able to do this since 2001 but advances in robotic technology have automated it, brought down costs and meant the new method can cope with high throughput whilst current methods are slow, labour intensive and expensive.

The team validated the method’s accuracy by comparing results from blood samples from 1,997 women taken at their 28-week antenatal visit with those from 1869 cord blood samples taken at delivery.

In 96% of cases, the correct RhD phenotype of the baby was predicted by the robotic genotyping tests. A false positive result was obtained in 0.8% (14 samples), and false negative in 0.2% (three samples). In 3.4% of cases results were either unobtainable or inconclusive.

If used as a guide to treatment, only 2% of the women would have received anti-RhD unnecessarily, rather than 38% without the genotyping tests.

The study could lead to the widespread screening of expectant mothers who are RhD negative as the test had high specificity and sensitivity and is feasible to be used on a large scale, comments Dr Sailesh Kumar, a consultant in fetal and maternal medicine at Queen Charlotte’s and Chelsea Hospital, London.

"If these techniques are shown to be as reliable earlier in pregnancy the arguments for recommending universal testing will be compelling," he said.

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