Follow-up Investigative Tests (Mother)
If a pregnant woman is found to have an unexpected clinically significant antibody, routine antenatal serologic tests on the mother include
- Antibody identification to detect clinically significant antibodies.
- Antigen typing: Once the antibody is identified, the mother is tested for the corresponding antigen, which she should lack.
- Antibody titration: Laboratories have different protocols. Depending on the antibody titer, titration may be performed at 2 or 4 week intervals after 18 weeks gestation.
- Maternal antibody titer is an unreliable indicator of fetal disease and is mainly done to determine if clinical fetal monitoring is warranted, e.g., Doppler ultrasonography of fetal cerebral blood flow or, more rarely, invasive monitoring such as amniocentesis.
- Careful quality control is needed for titrations. QC includes using red cells from donors with the same phenotype or likely genotype (e.g., R2r or R2R2) and titrating the new sample in parallel with the prior sample.
- A two-tube rise or more in a doubling dilution is considered a significant rise in titer.
- In the case of anti-D, a predetermined critical titer (often 16 or 32 for anti-D depending on the method) indicates the need for clinical fetal monitoring.