Q&A column

Q. Is it acceptable to perform weak D testing on a newborn who has an RhD-negative blood type and a positive direct antiglobulin test? We know a positive DAT might cause false-positive results on an Rh test, but can it cause false-negative results?

A.After an RhD-negative woman gives birth, newborn red blood cells found to be RhD-negative by direct agglutination are examined for the weak D phenotype using an indirect antiglobulin test for immunoglobulin G (IgG) anti-D. If the baby is weak D reactive, the mother is given Rh immune globulin. However, excess fetomaternal hemorrhage must be ruled out by quantifying the fetal red blood cells in the peripheral blood of the mother using the Kleihauer-Betke test or flow cytometry since the fetal rosette test is invalid for detecting weak D fetal red blood cells.¹

A reactive DAT on neonatal red blood cells is problematic relative to RhD typing for two reasons. First, simple IgG indirect antiglobulin test typing cannot be performed for a weak D phenotype because IgG is already present on the red blood cells. Second, if the mother has strong anti-D against RhD-positive fetal red blood cells, the baby’s RhD typing by direct agglutination might be falsely negative because of antigen blocking by anti-D.

Elution techniques are available to strip IgG from red blood cells while leaving the cells intact for typing. EDTA glycine acid reagents are most commonly used for this purpose.² After neonatal red blood cells are treated with such reagents and undergo a repeat DAT to make sure immunoglobulin is no longer present, they can be typed for RhD using routine direct and indirect antiglobulin testing.

1. Ramsey G, Park YA, Eder AF, et al. Obstetric and newborn weak D-phenotype RBC testing and Rh immune globulin management recommendations: lessons from a blinded specimen-testing survey of 81 transfusion services. Arch Pathol Lab Med. 2023;147(1):71–78.
2. Judd WJ, Johnson ST, Storry JR. Judd’s Methods in Immunohematology. 4th ed. AABB Press; 2022.

Glenn E. Ramsey, MD
Professor of Pathology
Northwestern University Feinberg School of Medicine
Medical Director, Blood Bank
Northwestern Memorial Hospital
Chicago, Ill.
Chair, CAP Transfusion, Apheresis, and Cellular Therapy Committee