GP Connect Clinical feature by Dr. Dimitar Azmanov, MD, PhD, FRCPA, Genetic Pathologist, Head of Department of Diagnostic Genomics PathWest Laboratory Medicine WA; Clinical A/Professor, Division of Pathology and Laboratory Medicine, UWA Medical School.
Since early November 2024, Rhesus D Non-Invasive Prenatal Testing (RHD NIPT) has been available to all Rhesus D-negative pregnant women in Western Australia who have not been previously alloimmunised against the RhD blood group, eliminating the need to give anti-D immunoprophylaxis in some pregnancies.
Key points:
- Rhesus D blood group feto-maternal incompatibility is associated with a risk of severe haemolytic disease of the fetus and newborn (HDFN).
- Routine prophylactic use of RhD Immunoglobulin (anti-D) in pregnancy care is in place to reduce the incidence of alloimmunisation (sensitisation) against RhD.
- RhD-negative women carrying a RhD-negative fetus(es) are not at risk of alloimmunisation and do not require anti-D immunoprophylaxis.
- MBS funded RHD NIPT is now routinely available to RhD-negative pregnant women to assess fetal RhD status and enable targeted anti-D immunoprophylaxis.
- Routine RHD NIPT is:
– only for RhD-negative pregnant people with no known anti-D antibodies
– done once in each pregnancy to determine fetal Rhesus blood genotype
– available through PathWest between 20 – 32 weeks’ gestation (ideally 20 – 24 weeks) and requires a specific request form. - Management of results:
– If RHD NIPT identifies fetal RHD genotype is also RhD-Negative, no anti-D immunoprophylaxis is required during this pregnancy
– If RHD NIPT identifies fetal RHD genotype is RhD-Positive, anti-D prophylaxis is required during this pregnancy as per usual guidelines. - For sensitising events occurring in RhD-negative pregnant people before RHD NIPT has been performed, manage as per usual guidelines.
Rhesus D blood group considerations in pregnancy care
HDFN is usually caused by maternal blood group Rhesus D (RhD or D-antigen) alloimmunisation and transplacental transfer of anti-D antibodies from a Rhesus D-negative mother to a Rhesus D-positive fetus1. The frequency of RhD-negative serotype is up to 15-17 per cent in individuals with European descent, but lower in other population backgrounds2.
Routine prophylactic use of RhD Immunoglobulin (anti-D) in pregnancy care
Routine anti-D immunoprophylaxis is administered at 28- and 34-weeks’ gestation to Rhesus D-negative pregnant women with no pre-formed anti-D antibodies. A dose of anti-D is also offered to RhD-negative women soon after sensitising events where the fetal RhD status is unknown or positive, and after delivery of an RhD-positive baby based on cord blood or neonatal RhD typing. Additional anti-D administrations may also be required for ongoing uterine bleeding or where otherwise indicated.
Current guideline: National Blood Authority: Guideline for the prophylactic use of Rh D immunoglobulin in pregnancy care
Utility of RHD NIPT testing
Up to 40 per cent of RhD-negative women carry a RhD-negative fetus and, therefore, are not at risk of alloimmunisation and receive anti-D unnecessarily3. The fetal RHD genotype can be assessed by non-invasive prenatal testing (NIPT) of cell-free fetal DNA (cffDNA) circulating in the maternal plasma. Anti-D immunoprophylaxis can then be targeted based on fetal genotype and only administered when the fetus is known to be RhD-positive in a RhD-negative mother. As the cffDNA is cleared from the maternal circulation soon after birth, RHD NIPT is current pregnancy-specific and needs to be repeated in subsequent pregnancies.
Additional information on RHD NIPT in pregnancy: Australian & New Zealand Society of Blood Transfusion: RHD In Pregnancy
RHD NIPT in Western Australia
Western Australia was the first State in Australia to implement RHD NIPT. The state-wide rollout began in early November 2024, coinciding with the Medicare update for item number 73420. RHD NIPT is available to all Rhesus D negative pregnant women who have not been previously alloimmunised against RhD.
RHD NIPT has been validated at PathWest Laboratory Medicine WA (PathWest) for gestational age 20 to 32 weeks’ gestation with test sensitivity of 100 per cent, false positive rate of 2.6 per cent, and 0 per cent false negative rate for cffDNA fraction greater than or equal to 2 per cent . The test is performed at Diagnostic Genomics, PathWest, and requires a specific request form to include expected date of delivery, number of fetuses, and maternal BMI. Results are usually available within two weeks. The high accuracy of the test is due to the measurement of the fetal fraction of cell-free DNA in the maternal plasma. Alternative tests in other jurisdictions (or overseas) do not measure the fetal fraction of cell-free DNA and it should be noted that false negative results (incorrectly determined fetal genotype as RHD-negative) have been reported in meta-studies to account for approximately 0.34 per cent of cases4. False negative results increase the risk of sensitisation if the recommended anti-D immunoprophylaxis has not been given.
PathWest resources:
Peer-reviewed references:
- Flegel, W.A. Molecular genetics and clinical applications for RH. Transfus Apher Sci 44, 81-91 (2011).
- Clausen, F.B. Lessons learned from the implementation of non-invasive fetal RHD screening. Expert Rev Mol Diagn 18, 423-431 (2018).
- Gordon, L.G. et al. Noninvasive fetal RHD genotyping of RhD negative pregnant women for targeted anti-D therapy in Australia: A cost-effectiveness analysis. Prenat Diagn 37, 1245-1253 (2017).
- Yang, H. et al. High-throughput, non-invasive prenatal testing for fetal rhesus D status in RhD-negative women: a systematic review and meta-analysis. BMC Med 17, 37 (2019).