Background

The clinical relevance of monocytosis in sickle cell disease (SCD) is uncertain. The purpose of this study was to explore the association of monocyte counts with clinical and laboratory variables in SCD.

Design and methods

This cross-sectional study was performed using a cohort of adult patients with SCD and control subjects without SCD. Complete blood counts, markers of hemolysis, coagulation activation, endothelial injury, and other laboratory studies were obtained in patients with SCD. Clinical complications were ascertained at the time of evaluation and tricuspid regurgitant jet velocity was determined by Doppler echocardiography.

Results

One hundred and fifty-seven patients with SCD and 24 healthy, African-American, control subjects were evaluated. The absolute monocyte count was increased in SCD patients compared with healthy controls (0.6 × 10⁹/l vs. 0.4 × 10⁹/l, P = 0.0025), with higher counts in HbSS/HbSβ⁰ thalassemia compared with HbSC/HbSβ⁺ thalassemia. In univariate analyses, absolute monocyte count was correlated with reticulocyte count, absolute neutrophil count, indirect bilirubin, lactate dehydrogenase, and inversely correlated with hemoglobin in SCD patients. Furthermore, monocyte count was correlated with soluble vascular cell adhesion molecule-1 (VCAM-1) in HbSS/HbSβ⁰ thalassemia patients. No significant associations were observed between absolute monocyte count and evaluated clinical complications. In multivariable analyses, reticulocyte count and absolute neutrophil count were significantly associated with absolute monocyte count.

Conclusions

Monocytosis is associated with hemolysis and inflammation in SCD. We suggest that hemolysis and the resultant erythropoietic response contribute to monocyte activation.