Quantification of sickle cells in the peripheral smear as a marker of disease severity

Blinded readers examined peripheral smears of 108 children with steady sickle cell (SC) disease and controls by counting ten 100× microscope fields and calculating percent of irreversible and reversible SC from total red cell population SC index (SCI). SCI was correlated to disease severity, and transfusion, hydroxyurea, or neither. Controls had a mean of 0.28% SC (range 0-0.64). Children with hemoglobin SS had a mean SCI of 5.12% ± 5.37 (range 0-30). SCI increased 0.33% with each increasing year (p <0.0001). Patients with SCI > 0.64 were 3.32 times as likely to experience clinical complications (p = 0.0124). Although blood transfusions and hydroxyurea decreased percent of SC, 72% treated patients had SCI >0.64, correlating with persistent sickling. This standardized method quantifies SC in peripheral smears. Percent of SC increased with age and correlated with disease severity, especially hemolytic complications, providing readily available information with minimal or no extra cost.