Absolute Reticulocyte Count Is Negatively Correlated with Fetal Hemoglobin during Infancy and Early Childhood in Patients with Sickle Cell Anemia

Hemoglobin switching is largely complete in healthy infants by 6 months of age. In infants with sickle cell anemia (HbSS, SCA), reticulocytosis begins early in life as fetal hemoglobin (HbF) is replaced by sickle hemoglobin (HbS). Previous studies demonstrated that patients with an ARC greater than 200 K/uL during early infancy (60-196 days of age) were at the highest risk for SCA-associated events. ^1,2 The objective of this study was to determine if ARC is related to HbF levels in a cohort of pediatric SCA patients.

A convenience sample of 106 children with SCA between the ages of 1 month and 20 years who were not receiving hydroxyurea or monthly blood transfusions were enrolled in this observational study [42 (39.6%) less than 1 year of age (28-362 days old), 46 (43.4%) between the ages of 1 and 10 years, and 18 (17.0%) between 10 and 20 years old]. After consent and assent were provided, discarded peripheral blood was obtained during routine clinic visits at steady state and analyzed within 48 hours of collection and storage at 4⁰C. Steady state was defined as a sample drawn at least 30 days following an acute event and at least 60 days following a blood transfusion. Hematologic data, including ARC and HbF levels, were measured using CLIA approved methods. F-cells were enumerated by flow cytometry following intracellular staining with a fluorescent antibody directed against HbF. Correlations were calculated to determine the relationships of ARC with HbF, F-cells, and other hematologic data, while two-tailed t tests were used to compare means.

Initial studies compared groups based upon ARC greater than or equal to 200 K/uL (ARC≥200) during infancy because of the previously reported utility of this threshold as a predictive marker for SCA severity.¹ Over one third of the infants less than 1 year of age (n=16) had an ARC≥200. Mean HbF and F-cell levels were significantly lower in the ARC≥200 group when compared to the ARC<200 group (HbF: 29.9±10.9% vs. 53.5±17.6%, respectively, p=2.2E-05; F-cells: 83.5±13.2% vs. 96.6±5.7%, p=6.2E-05). Mean hemoglobin levels were also lower in the ARC≥200 group [8.1±1.4 g/dL vs. 9.5±1.6 g/dL (ARC<200), p=0.005]. Of the 22 (52.4%) infants who had a HbF level greater than 40%, only 2 (9.1%) had an ARC greater than 200K/uL.

Enrolled patients were also grouped according to age and comparisons were made between ARC and HbF or F-cell levels. HbF and F-cell levels were negatively correlated to ARC in the infant subgroup (r=-0.696, p=3.1E-07 and r=-0.795, p=0.000, respectively). HbF and F-cell levels from children between the ages of 1 and 10 years were inversely related to the ARC, but the correlation was less significant (r=-0.626, p=3.3E-06 and r=-0.538, p=1.2E-04, respectively). The inverse relationship was no longer present in the oldest group of patients (HbF vs. ARC r=-0.203, p=0.420 and F-cells vs. ARC, r=-0.258, p=0.302).

According to both analytical approaches described here, increased ARC is associated with decreased HbF and F-cell levels in infants with SCA. Less robust negative correlations are maintained through age 10 years, but no significant correlation was identified in adolescence and young adulthood. Overall, the data suggest that increased ARC levels may identify SCA infants who manifest a more rapid or greater loss of fetal hemoglobin during the later stages of the HbF-to-HbS switching phenomenon.

1. Meier ER, Byrnes C, Lee YT, et al. Increased reticulocytosis during infancy is associated with increased hospitalizations in sickle cell anemia patients during the first three years of life. PLoS One 2013; 8(8):e70794. doi: 10.1371/journal.pone.0070794.

2. Meier ER, Wright EC, Miller JL. Reticulocytosis and anemia are associated with an increased risk of death and stroke in the newborn cohort of the Cooperative Study of Sickle Cell Disease. Am J Hematol 2014 May 31; doi: 10.1002/ajh.23777. [Epub ahead of print]