by W. Crews, M.D., Medical Director of Laboratory Services, Carter BloodCare
Molecular genotyping for red blood cell (RBC) antigens has become routinely used for some blood donors and certain patient populations (patients with hemoglobinopathies, recently transfused patients, patients with warm autoantibodies or multiple alloantibodies, for example).
In contrast, molecular RHD genotyping for patients with a known weak D phenotype, or to determine if administration of RhIG is definitively needed for pregnant women, is rarely requested. In addition, many hospitals do not perform serological testing for weak D. While AABB standards require weak D testing of blood donors, weak D testing for patients is optional.
In an effort to update practice guidelines and establish nationwide uniform practice, in 2014, the College of American Pathologists Transfusion Medicine Resource Committee (CAP TMRC) concluded selective integration of RHD genotypes would reduce unnecessary administration of RhIG and decrease transfusion of Rh-negative RBCs in women with a weak D phenotype and recipients with weak D phenotypes respectively. A year later, in 2015, CAP and AABB formed a joint RHD genotyping work group that published identical recommendations as the CAP TMRC.
In summary, the work group recommended RHD genotyping be done on pregnant women, newborns and potential transfusion recipients with a serologic weak D phenotype. Any patient whose RHD genotype is predicted to be weak D type 1, 2 or 3 should be treated as RhD positive for purposes of RhIG administration or selection of blood components for transfusion.
When should I order RHD genotyping?
- When there is a discrepancy in the patient’s Rh type
- When variable reactivity with multiple reagents is seen
- When Rh immediate spin is negative, but IAT is positive
- When Rh type is unknown