This investigation tests for the presence of fetal RHD in cell-free DNA (cfDNA) extracted from maternal blood. Cell-free fetal DNA (cffDNA) is present in maternal circulation from as early as 6 weeks’ gestation. Therefore tests can be devised to detect various fetal genes, including RHD.
Fetal RHD genotyping of cffDNA relies upon the detection of fetal alleles or genes,which are absent from the maternal genome. In Caucasians, the RhD-negative phenotype is most commonly due to a homozygous deletion of the RHD gene. At the IBTS qPCR (using hydrolysis probes) is used to amplify exons 7 and 10 of the RHD gene. Targeting this combination will detect RhD-positive, weak D and RHD variant genes, which results in a highly sensitive assay.
In African populations (or people of African decent), the majority of RhD-negative individuals are carriers of non-functional RHDΨ or RHD-RHCE hybrid genes. Although the RhD protein is not expressed in these individuals, RHD sequences present will be detected by our assay. Therefore these individuals will continue to receive RAADP. This exon combination was chosen maximise the sensitivity of the assay, thereby minimising false negative results.
Benefits and Indications for Testing
The Fetal RHD Screen is suitable for all pregnant RhD-negative women who DO NOT have alloanti-D.
The results obtained from the Fetal RHD Screen will allow clinicians and midwives to optimise the administration of anti-D. This is true for both anti-D given for any Potential Sensitising Events (PSEs) and second-trimester Routine Antenatal Anti-D Prophylaxis (RAADP).
The result of the Fetal RHD Screen can also be used to optimise the following:
- Anti-D quantification (women following RAADP administration)
- Cord RhD phenotyping
The IBTS has chosen to test booking samples to allow maximum benefit to be achieved through testing. Booking samples from all gestations will be accepted, however results will be issued as follows:
|Fetal RHD Screen||Gestation||Summary of Result Issued|
|RHD-positive||All||RHD-positive fetus; Anti-D recommended RHD-negative|
|RHD-negative||<11||Inconclusive; Anti-D recommended; Repeat sample|
|Inconclusive||All||Inconclusive; Anti-D recommended; Repeat sample|
|Maternal RHD||All||Maternal RHD; Anti-D recommended|