Cell typing the sensitized transfusion-dependent patient.

Extended red cell typing is required for the management of transfusion-dependent patients to confirm the identity of suspected alloantibodies or determine the specificity of potential additional antibodies that may be formed in the future. Typing may be complicated by the presence of circulating allogeneic cells or a positive direct antiglobulin test. Phenotyping such individuals by hemagglutination is dependent on the separation of a reticulocyte-enriched fraction by differential centrifugation. Flow cytometric typing of reticulocytes is also possible. The effectiveness of these techniques is limited in those who are heavily transfused or have low reticulocyte counts. Heavily transfused patients with sickle cell anemia may be typed, however, following hypotonic lysis of allogeneic cells. In patients with a positive direct antiglobulin test, sensitized cells are usually typed with either direct agglutinating antisera and/or IgG antisera following elution of the autoantibody. Inactivation of some antigens during the elution process or the lack of some antisera specificities limit such typing. By designing appropriate oligonucleotide primers, polymerase chain reaction (PCR) amplification of target gene sequences for most blood group systems and the identification of a large number of their allelic specificities is now possible. Peripheral blood leukocytes can be used as the DNA source. Restriction fragment length polymorphism determination is widely adopted for the identification of allelic specificity of the amplified target sequence. Alternate strategies, including allele-specific PCR, are often employed if the genetic basis of the polymorphism is more complex than a single nucleotide substitution, or if it does not create or ablate a restriction endonuclease cleavage site. These techniques may permit genotyping of sensitized transfusion-dependent patients, and can improve transfusion safety and efficacy.