D^u phenotyping by the rosette technique when the direct antiglobulin test is positive

The rosette technique provides a simple rapid and accurate procedure for D^u phenotyping of red blood cells (RBCs) with a positive direct antiglobulin test (DAT) due to antibodies other than those of the Rh system. We used Sebring and Polesky’s rosette technique to test RBCs with positive DATs. Characteristic microscopic agglutination was observed for the D^u phenotype which was different from the mixed-field agglutination seen as positive rosettes in feto-maternal hemorrhage (FMH). The microscopic “starry-sky” pattern typical for FMH-positive rosettes is easily differentiated from the uniform agglutination produced by the D^u phenotype. The rosette technique was performed on 202 Rh-negative cord RBCs out of 4815 cord cells with a positive DAT. Parallel testing for D^u by the chloroquine elution and antihuman globulin (AHG) test was done on 67 of the 202 cases. Four D^u RBCs that were identified by the rosette technique were confirmed by the chloroquine D^u technique. The rosette technique proved to be time saving and as sensitive as the routinely performed elution technique for removal of antibodies followed by the AUG D^u test.