A new protocol for RhD-negative pregnant women?

ILLUSTRATIVE CASE

A 30-year-old G1P0 woman presents to your office for routine obstetric care at 18 weeks’ gestation. Her pregnancy has been uncomplicated, but her prenatal lab evaluation is notable for blood type A-negative. She wants to know if she really needs the anti-D immune globulin injection.

Rhesus (Rh)D-negative women carrying an RhD-positive fetus are at risk of developing anti-D antibodies, placing the fetus at risk for HDFN (hemolytic disease of the fetus and newborn). If undiagnosed and/or untreated, HDFN carries significant risk of perinatal morbidity and mortality.²

With routine postnatal anti-D immunoglobulin prophylaxis of RhD-negative women who delivered an RhD-positive child (which began around 1970), the risk of maternal alloimmunization was reduced from 16% to 1.12%-1.3%.^3-5 The risk was further reduced to approximately 0.28% with the addition of consistent prophylaxis at 28 weeks’ gestation.⁴ As a result, the current standard of care is to administer anti-D immunoglobulin at 28 weeks’ gestation, within 72 hours of delivery of an RhD-positive fetus, and after events with risk of fetal-to-maternal transfusion (eg, spontaneous, threatened, or induced abortion; invasive prenatal diagnostic procedures such as amniocentesis; blunt abdominal trauma; external cephalic version; second or third trimester antepartum bleeding).⁶

The problem of unnecessary Tx. However, under this current practice, many RhD-negative women are receiving anti-D immunoglobulin unnecessarily. This is because the fetus’s RhD status is not routinely known during the prenatal period.

Enter cell-free DNA testing. Cell-free DNA testing analyzes fragments of fetal DNA found in maternal blood. The use of cell-free DNA testing at 10 to 13 weeks’ gestation to screen for fetal chromosomal abnormalities is reliable (91%-99% sensitivity for trisomies 21, 18, and 13⁷) and becoming increasingly more common.

A notable meta-analysis. A 2017 meta-analysis of 30 studies of cell-free DNA testing of RhD status in the first and second trimester calculated a sensitivity of 99.3% (95% confidence interval [CI], 98.2-99.7) and a specificity of 98.4% (95% CI, 96.4-99.3).⁷ Denmark, the Netherlands, Sweden, France, and Finland are using this method routinely. As of this writing, the American College of Obstetricians and Gynecologists (ACOG) has not recommended the use of cell-free DNA RhD testing in the United States, but they do note that as the cost of the assay declines, this method may become preferred.⁸ The National Institute for Health and Care Excellence in England recommends its use as long as its cost remains below a set threshold.⁹

This study evaluated the accuracy of using cell-free DNA testing at 27 weeks’ gestation to determine fetal RhD status compared with serologic typing of cord blood at delivery.