Footprints of Response to Hydroxyurea Are in the Hemogram.

The salutary effects of hydroxyurea (HU) in sickle dell anemia (SS) include decreased frequency of painful crises, decreased incidence of acute chest syndrome, decreased blood transfusion, and decreased mortality & morbidity. Markers of the response to HU include increased Hb F level, macrocytosis, improved RBC survival & deformability, and decreased WBC count. In this study we report markers of early response to HU by using mixed (random effects) models to determine the relationship between HU usage and the longitudinal trajectories of RBC parameters and the corresponding longitudinal trajectories of reticulocytes parameters.

RBC and reticulocyte parameters were determined on peripheral blood from selected patients (10 HU subjects and 11 placebo subjects) enrolled in the placebo controlled double blind multicenter study of hydroxyuea (MSH) in SS. Multiple blood samples were obtained during enrollment in the study at a rate of about once every two weeks. The mean weeks of follow-up from enrollment date was 86.6 weeks (range 2-172 weeks). RBC and Reticulocyte indices were measured on a Siemens H*3 RTX (H*3) hematology analyzer. Laser light scattering was used to quantify cell volume, Hb concentration, and the light absorbance of cells stained with oxazine 750 to detect reticulocytes and distinguish them from mature cells. A total of 20,000 cells were counted for each sample. After the volume (V) and Hb concentration (HC) of individual mature RBC and reticulocytes were measured, the Hb content (CH) of individual cells was calculated. Mixed (random effects) models were used to determine the relationship between the longitudinal trajectories of RBC and reticulocyte indices and HU usage.

Goodness-of-fit criteria for the mixed (random effects) models [Akaike Information Criterion (AIC), Akaike Information Criterion, Small Sample Size Corrected Version (AICC) and the Bayesian Information Criterion (BIC)] indicated that the RBC indices modeled more precisely than those of reticulocytes. However, both the RBC indices and reticulocytes provided useful information regarding the relationship between their longitudinal trajectories and HU usage. There was a significant relationship between HU usage and the following longitudinal trajectories: mean cell hemoglobin (CH) (p<.0001), reticulocyte mean cell hemoglobin (CHr) (p<.0001); Cell hemoglobin concentration (CHCM) (p=.0394); Reticulocyte mean cell volume (MCVr) (p=.0019); hemoglobin distribution width (HDW) (p=.0003); and reticulocyte hemoglobin distribution width (HDWr) (p=.0008). HU usage produced higher value trajectories than those generated by placebo usage for the following: CH, CHr; and MCVr. These changes were first detected 10 weeks after starting HU and before the increase in Hb F levels.

Together the data suggest that subtle and early markers of response to HU reside in the hemogram. This is a fast, simple and inexpensive methodology provided the lab in question has the capability to determine the indices of both reticulocytes and RBC.

No relevant conflicts of interest to declare.