by William Crews, MD
Red blood cell genotyping predicts the antigen phenotype of the red cell by analyzing the portion of DNA that codes for red cell proteins/antigens.
Carter BloodCare has performed red cell genotype testing for 10 years. Initially we used this testing to phenotype blood donors to increase our “rare unit” inventory. Over the years it became evident that genotyping could supplement traditional serologic testing for patients that required complex serological tests such as adsorptions and elutions.
While serology remains the gold standard for ABO and Rh typing, antibody screening, and compatibility testing for transfusion, genotyping can be utilized as part of pretransfusion testing for certain patients. In our reference lab we commonly perform genotyping for patients who require chronic transfusion, have an autoimmune hemolytic anemia, have an alloantibody to a high-prevalent antigen, have discrepant serology results or have recently been transfused. The benefits of genotyping these patients are detailed in the table below.
In fact, genotyping proved so beneficial that in 2015 we implemented a new protocol we termed Modified Pretransfusion Workup (modified workup). If we feel a patient can benefit from a modified workup, during the initial serological workup we perform an antibody screen, DAT, and if necessary, an elution and/or adsorption. Red cells are crossmatched, and genotyping is performed in case the patient was referred for future reference testing. If the patient requires transfusion on a subsequent admission the ABO type and DAT are repeated, if there is no change from previous results or strength of reaction, then no further serological testing is performed and antigen-matched units are issued in lieu of performing a crossmatch.
We feel the modified workup improves the quality of patient care by reducing the turnaround time of compatible red cells, allowing the patient to be transfused sooner. The modified workup also allows for significant decrease in the cost of pretransfusion testing since the reference staff does not have to repeat time-consuming procedures each time subsequent testing is requested on a previously genotyped patient.
Available tests for genotyping continue to grow. In November of 2017 we expanded our molecular test menu and began offering weak D genotyping. The weak D test can identify six variants of the RHD gene, and provides the predicted phenotype for RHD weak types 1,2, and 3. The test also predicts if a patient has a RHD deletion, or has the RHD Pseudogene.
Weak D genotyping is a great test to determine if a patient can safely be transfused Rh-positive red cells, or if administration of RhIg is necessary in women who are pregnant. We recommend weak D genotyping in the following situations:
• 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
The use of genotyping in the blood bank has steadily become more common, and has proven to be an invaluable supplement to serologic testing. The prediction of HPA-1a and HPA-1b phenotype testing is also now available, while this test is predominantly used for donors it can be utilized in the patient setting. With the continued development of specific tests available by genotyping, and the expected increase of information able to be obtained, genotyping will soon be essential instead of optional for donor and patient testing.