Non Invasive Screening for Fetal RHD-Genotype in All D-Negative Women Is Reliable and Cost-Effective.

About 40% of D-negative pregnant women receive antenatal anti-D despite a D-negative fetus. We have developed an automated assay for fetal genotyping using cell-free fetal DNA from maternal plasma to restrict antenatal prophylaxis to women carrying a D-positive fetus. This will save anti-D of which is worldwide shortage, and will prevent the unnecessary administration of a blood product. 1 ml of plasma is automatically presented (Tecan) to a DNA isolation-robot (Roche). The DNA-eluate is tested (after automatically pipetting) in triplicate in a real-time RHD exon7-PCR (ApplBios).

Results

Plasma from 2415 D-negative pregnant women, whose blood was sent in for 28–30^th week antibody screening, has been tested. In 35 cases (1.44%) the women (10 weak D and 25 normal D-positive) were typed as D-positive by repeated serological tests, and excluded from the study.

1465 of the plasma’s (61.59%) were typed RhD positive and 915 RhD negative. Because the RHD exon7-PCR will give positive results in D-negative women carrying D-negative variant RHD genes such as RHDΨ, we determined the incidence of these genes in our studygroup. In all cases (n=39) with Ct values below 33.2 DNA was isolated from maternal leukocytes. Of the 19 cases with extremely low Ct values (<32), 6 women carried an RHDΨ gene, 5 an RHD variant type VI, 3 weak D (type 1, 11 and 17) and 1 RHDdel. The 24 other women did not carry RHD genes.

To compare the plasma PCR results with cord blood serology, all women and obstetric caregivers were sent questionnaires on cord blood serology. 55% responded (n=1257). For cases with discrepant results(n=21) or incompleted questionnaires(n=31) the laboratory which performed the cord blood typing was contacted. Finally, concordant results were obtained in 1249 of the cases. In 3 cases the genotype suggests D-positivity while serology is D-negative. In 5 cases no RHD-sequences were detected in plasma but cord blood was typed D-positive. For the discrepant cases it cannot be concluded yet, which assay gave the correct phenotype.

Conclusion

This is the first large scale automated study demonstrating the feasibility of screening D-negative women to restrict antenatal anti-D to women carrying D-positive fetuses. For the recognition of women at risk for immunization, the plasma-PCR test seems to be at least as reliable as the serological test. Furthermore, postnatal prophylaxis could have been given directly after delivery in all women with a positive PCR-result, saving cord blood serology tests and possibly increasing the effectiveness of postnatal prophylaxis.

An economic evaluation demonstrated that this screening policy is cost effective in the Netherlands.