Utilization of a cell separation technique to evaluate patients with a positive direct antiglobulin test

Density distribution curves of red blood cells (RBC) from patients with a positive direct antiglobulin test (DAT) were compared to a standardized curve constructed from cell column measurements of centrifuged microcapillary tubes filled with RBC and phthalate ester mixtures encompassing a specific gravity range of 1.078 to 1.114. Shortened survival resulted in a loss of older RBC and a shift of the curve to the right over the high specific gravity ester range. Reticulocytosis resulted in a downward shift of the curve over the low specific gravity range. In patients with a positive DAT due to an autoantibody or drug, the density distribution curve was either normal or demonstrated evidence of shortened RBC survival. In patients with a positive DAT due to an alloantibody, however, evidence of shortened survival was not seen. The distribution of antibody on young and old RBC harvested from the appropriate microcapillary tubes depended upon the etiology of that antibody. In patients with a positive DAT due to to an alloantibody or drug, both the young and old RBC gave an equally reactive DAT, while in patients with a positive DAT due to an alloantibody the young cells were weakly reactive or nonreactive and the older cells were more strongly reactive. When used together, the position of the density distribution curve and the pattern of distribution of antibody coating on young and old RBC provide important diagnostic information about the etiology and clinical status in a patient with a positive DAT and allow for the recognition of an alloantibody and autoantibody when both are present in the same patient.