Red cell size and the clinical and haematological features of homozygous sickle cell disease.

The contribution of red cell size (mean cell volume) to the clinical and haematological manifestations of homozygous sickle cell (SS) disease has been investigated by comparing the features of two groups of patients with low (less than or equal to 80 fl) and high (greater than 95 fl) MCV values after matching for age, sex and fetal haemoglobin level. The microcytic group manifested significantly higher Hb, PCV, RBC and HbA2 levels and significantly lower reticulocyte and irreversibly sickled-cell counts. Clinical features were not less severe in the microcytic group, splenomegaly persisting for longer and painful crises were more common although the latter difference did not reach significance. The milder haematological picture associated with decreased intravascular sickling was not reflected in a more mild clinical course. It is postulated that the higher viscosity accompanying the higher haemoglobin levels in microcytic patients may offset the rheological advantages of decreased intravascular sickling.