Mixed field reactions in ABO and Rh typing chimerism likely resulting from twin haematopoiesis.

Abstract

Discrepancies in ABO and Rh typing due to the presence of mixed field reactions may be encountered in the transfusion medicine laboratory1,2. In order to resolve the discrepancy and to provide compatible blood for transfusion, there is a requirement to obtain relevant historical information from the patient and to perform additional laboratory investigations. Often the cause of the mixed field reactions is easily ascertained on history, when the patient is found to have had recent transfusion or stem cell transplantation from a non-group identical donor. Certain ABO subgroups and pathologic or physiologic conditions may also lead to mixed field reactions on ABO typing; however, they should rarely affect Rh typing1,2. Similarly, a change in RhD phenotype has been described in a patient with leukaemia, but this did not affect ABO typing3. One of the potential causes of mixed field reactions on ABO and Rh typing is the presence within an individual of a chimeric state or mosaicism4,5. A chimera is present when two or more distinct cell populations containing genetic material from more than one zygote exist within an individual. Although descriptions highlighting the concept of chimerism as a cause of blood group typing discrepancies have existed for decades, chimerism is uncommon and presents challenges when mixed-field agglutination is encountered on forward or reverse ABO typing during routine pre-transfusion testing6,7. There are several forms of chimerism which have varied manifestations1,4,5. Congenital chimerism refers to the situation where there is either embryo fusion or dizygotic twin-twin blood vessel anastomoses between two dichorionic placentas which results in the exchange of haematopoietic cells. Partial haematopoietic chimerism also exists and is the result of transfusion or stem cell transplantation; in these situations evidence of chimerism is confined to one body compartment (haematopoietic cells). Both serologic and molecular methodologies may be useful in determining the cause of mixed field reactions. In addition, a highly sensitive flow cytometric method may assist in characterizing ABO subgroups through the detection of low levels of A and B antigen on red blood cells (RBCs) or on low fractions of RBCs in a sample8. Here we describe the use of blood group genotyping and flow cytometry to investigate a patient with mixed field reactivity in ABO, RhD, and RhE typing.