High-throughput non-invasive prenatal testing for fetal RHD genotype

NICE technology appraisal guidance on routine antenatal anti-D prophylaxis for women who are rhesus D negative recommends anti‑D immunoglobulin for all rhesus‑D (D) negative pregnant women who are not known to be sensitised to the D antigen, to reduce the risk of sensitisation. The British Committee for Standards in Haematology (BCSH) guideline on anti-D immunoglobulin to prevent haemolytic disease of the fetus and newborn also recommends that all D‑negative pregnant women who are not known to be sensitised to D antigen have anti‑D immunoglobulin after:

• potentially sensitising events

• birth, if the baby is confirmed to be D positive by cord blood typing.

Anti‑D immunoglobulin is produced from the pooled plasma donated by large numbers of D‑negative people who have had a transfusion of D‑positive red cells to stimulate the production of D antibodies. It is a finite resource and, because there have been shortages in the past, it needs to be used carefully to maintain stocks. Anti‑D immunoglobulin is a blood product and may also carry the risks common to all blood products, including physiological reactions, processing errors and the potential future risk of unknown blood-borne viruses or prion diseases. Physiological reactions must be reported to the Medicines and Healthcare products Regulatory Agency and processing errors to the Serious Hazards of Transfusion Scheme.

High-throughput non‑invasive prenatal testing (NIPT) for fetal RHD genotype involves analysing cell-free fetal DNA in maternal blood and is intended for use in pregnant women who are D negative and are not sensitised to D antigen. It is a laboratory-developed test offered by the International Blood Group Reference Laboratory, Bristol. This laboratory is an accredited NHS Blood and Transplant Laboratory that is currently providing NIPT for RHD genotype for some NHS patients.

High-throughput NIPT for fetal RHD genotype would allow D‑negative women who are carrying a D‑negative fetus to avoid unnecessary treatment with anti‑D immunoglobulin.

High-throughput NIPT for fetal RHD genotype would allow D‑negative women to make an informed choice about whether to have treatment with anti‑D immunoglobulin. This may improve adherence to anti‑D immunoglobulin treatment, reduce the number of sensitisations and so reduce haemolytic disease of the fetus and newborn in later pregnancies.

During pregnancy, small amounts of fetal blood can enter the maternal circulation (an event called fetomaternal haemorrhage). The presence of fetal D‑positive cells in the maternal circulation, after fetomaternal haemorrhage, can cause a mother who is D negative to produce antibodies against the D antigen on the fetal blood cells (anti‑D) – a process called sensitisation. Sensitisation can happen at any time during pregnancy, but is most common during the third trimester and delivery. It can follow events in pregnancy known to be associated with fetomaternal haemorrhage, such as medical interventions, terminations, late miscarriages, antepartum haemorrhage and abdominal trauma. These are called potentially sensitising events.

The process of sensitisation has no adverse health effects for the mother and usually does not affect the pregnancy during which it occurs. However, if the mother is exposed to the D antigen from a D‑positive fetus during a later pregnancy, the immune response is quicker and much greater. The anti‑D produced by the mother can cross the placenta and cause haemolytic disease of the fetus and newborn. This can cause severe fetal anaemia, leading to fetal heart failure, fluid retention and swelling (hydrops), and intrauterine death.

The risk of sensitisation can be reduced if D‑negative pregnant women have anti‑D immunoglobulin. Before anti‑D immunoglobulin was available, the incidence of sensitisation in D‑negative women after the birth of 2 D‑positive babies was about 16%. Haemolytic disease of the fetus and newborn, which occurred in about 1% of all births, was a significant cause of morbidity and mortality. After routine postpartum anti‑D prophylaxis was introduced, the incidence of D sensitisation dropped to about 2%. The sensitisation rate has further reduced since the introduction of routine antenatal anti‑D prophylaxis.

In England, there were 646,904 births from April 2013 to March 2014, of which about 15% (97,036 births) were to D‑negative women. About 40% of these women carry a D‑negative fetus (around 39,000 per year) and so do not need to have anti‑D immunoglobulin. D‑negative status occurs in about 15% of people of white European family origin, about 3% to 5% of people of black African family origin, and is very rare in people of eastern Asian origin. Most D‑negative people of white European family origin have an RHD gene deletion; less than 1% have RHD gene variants. However, in D‑negative people of black African family origin, 66% have an inactive RHD gene (the RHD pseudogene), which mostly results from genes that contain D sequences but do not produce D antigen.