Reassessment of Critical Anti-D Antibody Titer in RhD Alloimmunized Antenatal Women

In the setting of RhD-alloimmunized pregnancy, laboratory variations in critical titer (CT) of anti-D antibody may result in needless referrals or a compromised fetal outcome.

RhD-alloimmunized pregnant women were included. Fetal outcome was categorized based on cord hemoglobin and interventions required. For 3 commonly used CTs of 8, 16, and 32, sensitivity and specificity as well as positive and negative predictive values were computed.

When compared with CTs of 16 and 32, we detected 6.9% and 19.4% more cases of moderate-severe hemolytic disease of the fetus and newborn by using 8 as the CT. However, this leads to greater rate of unnecessary referral (12.1%, 10/82) than a CT of 16 (8.2%, 6/73) and 32 (4.9%, 3/61). A CT of 8 demonstrated 100% sensitivity, but 12.1% (10/82) of patients were referred needlessly.

Because of its 100% sensitivity, we advocate decreasing the CT to 8. However, this may lead to unwarranted referrals.