Resolving Rh status of a newborn infant whose direct antiglobulin test is positive

A blood banker in Colorado who wrote that his hospital delivers about 2000 babies a year, and infants born to Rh-negative mothers are tested for Rh type, including a test for weak D. In some situations, such as ABO incompatibility where there is a positive direct antiglobulin test, an umbilical cord blood sample may test equivocal for Rh type. The inquiring blood banker reports that his lab treats these umbilical cord blood samples as "possible" weak D positive and they administer Rh immunoglobulin to the mother. They inform the obstetrician and pediatrician that the neonate's Rh type cannot be determined with certainty at this time, and suggest repeat testing of the infant after a few months. The problem, according to the Colorado blood banker, is that his laboratory has been routinely performing rosette tests on maternal samples to rule out excessive fetal-maternal hemorrhage, but that now, according to the new AABB Practice guidelines for prenatal-perinatal immunohematology revisited, "if the infant is of a weak D phenotype, the mother is a candidate for RhIG and should be evaluated for excessive FMH." The guideline continues, "In this setting it is inappropriate to use the rosette test or any other anti-D based method to assess for excessive FMH (number of D sites on fetal RBCs will be low, resulting in false-negative tests). Rather, methods should be used that detect hemoglobin F in fetal RBCs". The inquiring Colorado blood banker wonders what others are doing, when faced with a situation similar to that described above.

Before sending this question to the full e-network for discussion, the above scenario was reviewed by an acknowledged expert in the field of prenatal-perinatal immunohematology. According to W. John Judd, FIBMS, MIBiol, Professor of Immunohematology, Department of Pathology, University of Michigan Medical Center, who was involved in the development of the Practice Guideline referenced above, "when we have a newborn whose RBCs have a positive direct antiglobulin test (DAT) due to fetal/maternal ABO incompatibility such that we cannot obtain a valid test for weak D, we treat the RBCs with EDTA-Glycine (EGA) which is available commercially, and repeat the test for weak D (including appropriate controls). This approach invariably resolves the issue as far as the Rh status of the infant and the maternal RhIG candidacy. We have data that show that EGA treatment does not diminish our ability to detect weak expression of D."

Editor's NOTE: For those interested in pertinent discussions on this issue, please visit the following links:

• Routine cord blood serologic testing - Who needs it? 
• Practice guidelines for prenatal and perinatal immunohematology, revisited (Transfusion, Nov., 2001)

Submit comments to the e-Network Forum at enetworkforum@cbbsweb.org

Ira A. Shulman, MD
CBBS e-Network Forum Senior Editor & Moderator

W. Tait Stevens, MD
CBBS e-Network Forum Editor & Moderator

Elizabeth M. St. Lezin, MD
CBBS e-Network Forum Associate Editor & Moderator