Which laboratory test should be ordered for an Rh‑negative pregnant woman to screen for allo‑antibodies?

Antibody Screening for Rh-Negative Pregnant Women

Order an indirect antiglobulin test (IAT), also known as the indirect Coombs test, to screen for alloantibodies in Rh-negative pregnant women.

Standard Screening Protocol

• All pregnant women, regardless of Rh status, should be typed and screened for alloantibodies with an indirect antiglobulin test at the first prenatal visit and again at 28 weeks gestation. 1

• The indirect antiglobulin test detects maternal antibodies against fetal red blood cell antigens that could cause hemolytic disease of the fetus and newborn in current or subsequent pregnancies. 2

Timing of Antibody Screening

• Perform the initial antibody screen at the first prenatal visit to identify any pre-existing alloimmunization from previous pregnancies, transfusions, or other sensitizing events. 1

• Repeat the antibody screen at 28 weeks gestation, which coincides with the timing of routine antenatal RhIg prophylaxis administration. 1

• If a potentially sensitizing event occurs (bleeding, trauma, invasive procedure), antibody testing should be performed at least 4 weeks after the exposure to allow sufficient time for antibody development if sensitization occurred. 3

Important Clinical Considerations

• The indirect antiglobulin test will detect anti-D antibodies from RhIg prophylaxis administration, which is expected and does not indicate true sensitization. After antepartum RhIg at 28 weeks, 85-97% of women will test positive for anti-D antibodies for 4-11 weeks, with titers typically ranging from 1:8 to 1:16. 4

• When anti-D is detected after RhIg administration, this represents passive antibody from the prophylaxis, not maternal alloimmunization—postpartum RhIg should still be administered if the infant is Rh-positive. 4

• The test screens for all clinically significant alloantibodies, not just anti-D, including antibodies to other Rh antigens (C, c, E, e), Kell, Duffy, Kidd, and other blood group systems. 1

Special Situations

• At the time of abortion or pregnancy loss, blood type and antibody screen should be performed unless recent results from the current pregnancy are already available. 1

• Women with "weak D" (formerly Du-positive) phenotype should be managed as Rh-positive and do not require RhIg or special antibody monitoring. 1

• If paternity is certain, Rh testing of the baby's father may be offered to eliminate unnecessary RhIg administration if he is Rh-negative, though antibody screening should still be performed per routine protocol. 1