Interpretation of pretransfusion testing in obstetrical patients who have received antepartum Rh immunoglobulin prophylaxis.

BACKGROUND AND

OBJECTIVES

Determining whether anti-D represents active or passive alloimmunization after RhIg administration is challenging. The objectives were to use antibody reaction strength to differentiate patients who may have become RhD alloimmunized during pregnancy from those manifesting passive anti-D and to investigate which methods work best for this determination.

MATERIALS AND

METHODS

Data were collected from patients residing in the Edmonton region of Canada, ≥18 years old, undergoing antibody screening in late pregnancy, who received 300 μg (1500 IU) of RhIg in the preceding 120 days. A total of 1106 tests were performed on 1050 blood samples from 963 patients: 640 by PEG, 156 by gel-card and 310 by solid-phase methodology.

RESULTS

PEG was the least sensitive to passive anti-D, with significantly fewer positive results at ≥8 weeks after RhIg compared to the other methods. Strength of reactivity and time since RhIg injection could be used to identify patients at high risk using PEG as a 4+ reaction at any time, ≥3+ at >2 weeks, ≥2+ at >6 weeks and ≥1+ at >14 weeks. Similarly, the gel-card method thresholds were 4+ at >5 weeks, ≥3+ at >10 weeks and ≥2+ at >15 weeks. Reaction strength by solid-phase was too variable to establish useful thresholds by this method. Infant RhD status did not significantly affect results.

CONCLUSION

Patients can be risk stratified for alloimmunization by anti-D reaction strength and time after RhIg administration. The PEG method was the best of those investigated, but the gel-card method can also be used.