Correspondence:

Recent evidence suggests that red blood cell aggregation and the ratio of hematocrit to blood viscosity (HVR), an index of the oxygen transport potential of blood, might consid-erably modulate blood flow dynamics in the microcirculation. It thus seems likely that these factors could play a role in sickle cell disease. Design and Methods We compared red blood cell aggregation characteristics, blood viscosity and HVR at dif-ferent shear rates between sickle cell anemia and sickle cell hemoglobin C disease (SCC) patients, sickle cell trait carriers (AS) and control individuals (AA). Results Blood viscosity determined at high shear rate was lower in sickle cell anemia (n=21) than in AA (n=52), AS (n=33) or SCC (n=21), and was markedly increased in both SCC and AS. Despite differences in blood viscosity, both sickle cell anemia and SCC had similar low HVR values compared to both AA and AS. Sickle cell anemia (n=21) and SCC (n=19) sub-jects had a lower red blood cell aggregation index and longer time for red blood cell aggre-gates formation than AA (n=16) and AS (n=15), and a 2 to 3 fold greater shear rate require