A patient with homozygous sickle cell disease showed a reduced incidence of painful crises as a result of regular exchange transfusion, but on three occasions when transfusion treatment was interrupted, a painful crisis occurred. Onset of painful crisis was associated with raised packed cell volume (PCV) or percentage of haemoglobin S (HbS%), or both. Measurement of whole blood viscosity using in vitro mixtures of blood group compatible normal (AA) and sickle (SS) cells showed that above an HbS of 25% any increase in PCV caused a disproportionate increase in whole blood viscosity. These clinical observations and laboratory data suggest that when regular exchange transfusions are terminated both HbS% and PCV should be carefully monitored. Prophylactic venesection should be considered for patients who maintain their PCV after transfusion as HbS% rises.
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