title: Interpretation of pretransfusion testing in obstetrical patients who have received antepartum Rh immunoglobulin prophylaxis.
abstract: OBJECTIVE
  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.
  
  
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: The Department of Laboratory Medicine and Pathology, Alberta Health Services, Edmonton, AB, Canada.
authors.forename: A J
authors.initials: AJ
authors.lastname: Szkotak
authors.affiliation: The Department of Laboratory Medicine and Pathology, Covenant Health, Edmonton, AB, Canada.
authors.forename: B
authors.initials: B
authors.lastname: Lunty
authors.affiliation: The Department of Laboratory Medicine and Pathology, Alberta Health Services, Edmonton, AB, Canada.
authors.forename: S
authors.initials: S
authors.lastname: Nahirniak
authors.affiliation: The Department of Laboratory Medicine and Pathology, Alberta Health Services, Edmonton, AB, Canada.
authors.forename: G
authors.initials: G
authors.lastname: Clarke
mesh_terms: D000328:Adult; D002170:Canada; D005260:Female; D006801:Humans; D007159:Immunologic Tests / Q000379:methods*; D011247:Pregnancy; D012203:Rh Isoimmunization / Q000453:epidemiology / Q000517:prevention & control*; D018029:Rho(D) Immune Globulin / Q000008:administration & dosage / Q000009:adverse effects* / Q000276:immunology / Q000627:therapeutic use; D012680:Sensitivity and Specificity
chemical_list: C061961:RHO(D) antibody; D018029:Rho(D) Immune Globulin
keywords: RBC antigens and antibodies; haemolytic disease of the foetus and newborn; immunohaematology; proficiency testing; serological testing; transfusion medicine