Anti-RH1 alloimmunization: At what maternal antibody threshold is there a risk of severe fetal anemia?

To define a threshold of maternal antibodies at risk of severe fetal anemia in patients followed for anti-RH1 alloimmunization (AI).

We conducted a retrospective study of patients followed for anti-RH1 AI at the Lille University Hospital. The first group, severe anemia, included patients who received one or more in utero transfusions (IUT) or who were induced before 37 weeks of pregnancy for suspected severe fetal anemia. The second group, absence of severe anemia, corresponded to patients without intervention during pregnancy related to AI. Sensitivities, specificities, and positive and negative predictive values for screening for severe fetal anemia were calculated for the antibody thresholds of 3.5 and 5 IU/ml for the quantification.

Between 2000 and 2018, 207 patients were included 135 in the severe anemia group and 72 in the no severe anemia group. No severe anemia was observed for an antibody titer below 16. For an antibody threshold of 3.5 IU/ml, the sensitivity was 98.2%, with 30.2% false positives. All severe anemias were detected in the second trimester; two cases of severe anemia were not detected in the third trimester. For an antibody threshold of 5 IU/ml, the sensitivity was lower at 95.6%, with five cases of severe anemia not detected.

The antibody threshold of 3.5 IU/ml for the quantification and 16 for the titration allow targeting patients requiring close monitoring by an experienced team in case of anti-RH1 AI.