Erythrocyte volume distribution analysis and hematologic changes in two horses with immune-mediated hemolytic anemia

Immune-mediated hemolytic anemia was diagnosed in two horses on the basis of regenerative anemia, increased erythrocyte fragility in hypotonic saline, autoagglutination, and a positive direct antiglobulin (Coomb's) test. During steroid therapy partial resolution of the anemia was indicated by rising packed cell volume, macrocytosis, and bone marrow erythroid hyperplasia. Using erythrocyte volume distribution histograms (erythrograms), the regenerative response was characterized by analysis of macrocytic and normocytic erythrocyte subpopulations. In both horses, a gradual net increase of about 2 X 10(6) macrocytes/microliter occurred over a four- to five-week period. Over the same interval there was a gradual decrease in the number of normocytes. We suggest that the macrocytes remained large through this period rather than contributing to normocyte population growth. Erythrograms may provide an additional means of evaluating erythrocyte regeneration in horses.