In some cases, samples are encountered that contain two distinct, separable populations of RBCs. Usually a mixture occurs because group O RBCs were transfused to a group A (or group B) patient. RBC mixtures also occur in a condition called Chimerism?, resulting either from the intrauterine exchange of erythropoietic tissue by fraternal twins or from mosaicism? arising through dispermy. Less frequently, it occurs when a patient has received a transplant of bone marrow that is of an ABO group different from the patient's own.
Mixed-field agglutination is characteristically seen when A3 RBCs are tested with anti-A. If the agglutinated RBCs are removed and the remaining RBCs again tested with anti-A, mixed-field agglutination occurs in the residual population as well. Mixed-field agglutination may also be seen with RBCs carrying A antigens weakened by diseases such as leukemia or with Tn? RBCs.