Routine non‐ABO blood group antigen genotyping in sickle cell disease: the new frontier in pretransfusion testing?

A lloimmunization to blood group antigens remains an important and highly relevant clinical problem. Upwards of 3% of general, hospital-based patients possess a detectable underlying alloantibody, with higher alloimmunization rates reported in chronically transfused patients. Among this latter group, individuals with sickle cell disease (SCD) have been found to be among the most alloimmunized of transfused patients, with rates of alloantibody development ranging from 20% to nearly 80%. Historically, much importance has been placed on the logistical complexities associated with locating compatible red blood cell (RBC) products for highly alloimmunized patients. However, it has become increasingly apparent that non-ABO alloantibodies are a significant cause of morbidity and mortality, both in the United States and abroad. Individuals with SCD, who are dependent on RBC transfusion as a cornerstone of their care, appear particularly prone to the harmful effects of RBC alloimmunization. Not only do alloimmunized patients with SCD suffer morbidity due to delays in finding compatible products for transfusion, but RBC alloantibody evanescence puts these individuals at risk of severe and life-threatening delayed hemolytic transfusion reactions upon reexposure to the immunizing antigen. Given the high rates of blood group alloimmunization seen in SCD, and its associated hazards for these patients, prophylactic matching of non-ABO antigens between SCD recipients and RBC donors has been proposed as a means to reduce antibody development and limit adverse events. In fact, a recent publication on evidence-based management of SCD composed by the NIH recommended that RBC units administered to individuals with SCD be matched for the C, E, and K antigens. As such, some institutions have adopted the practice of partial matching (i.e., C, E, and K) or extended matching (i.e., C/E/K plus other antigens within the Rh, Duffy, Kidd, and MNS systems) for their transfused SCD patients. Traditionally, non-ABO antigen matching has been accomplished using serologic (i.e., antibody-based) methods. However, such approaches are “low throughput” in that they are labor-intensive and often cannot be efficaciously utilized for large numbers of blood group antigens. Moreover, serologic methods have some inherent limitations that may be especially relevant in SCD populations: 1) difficulties in distinguishing between donor and recipient RBCs for recently transfused patients, in the absence of reticulocyte separation or hypotonic cell lysis, 2) insensitivity to minor structural variations for a given antigen that are clinically relevant from an immunogenicity standpoint, and 3) lack of availability of commercial reagents for some antigens. Therefore, the blood bank community has looked toward other methods of blood group determination. In this regard, DNA-based tests have been extensively studied for assessing blood group antigen status in a “highthroughput” manner, while simultaneously overcoming many of the limitations associated with serologic methods. Genetic assays thus represent an exciting new frontier for pretransfusion testing in SCD. To date, however, there have been virtually no reports on the use of genotyping to replace serologic non-ABO antigen phenotyping for routine sickle cell–associated transfusion practice. In this issue of TRANSFUSION, Casas and colleagues report their experience using DNA-based testing of single-nucleotide polymorphisms to predict nonABO blood group antigens in a large cohort of patients with SCD. The authors carefully compared the results of their genotyping assays with serologic phenotyping records in an effort to probe for discrepancies in their DNA-based approach. They found a high concordance rate at 0.9997 of all antigenic comparisons. In discrepant cases, the vast majority of individuals with repeat serologic testing (64/66; 97%) showed agreement with genotypic results, indicating that the initial serologic result was likely unreliable. The results of this study have important implications for transfusion practice. First, this study helps to answer one of the significant concerns that has plagued genetic testing for blood group antigen prediction: the worry that genotype may not always equal phenotype for clinically significant antigens, especially in diverse patient populations such as those with SCD. Casas and colleagues have convincingly shown, via their large SCD cohort, that the concordance rate between genetic and serologic-based approaches is sufficiently high to put such worries to rest. Moreover, the authors found that genotypic testing was particularly helpful in evaluating complex antibodies and improving donor selection for high-incidence antigens such as U and hr. Their doi:10.1111/trf.13065