Reticulocyte subpopulations and reticulocyte maturity index (RMI) rise as body iron status falls

To investigate the influence of body iron status on reticulocyte subpopulations and reticulocyte maturity index (RMI), we measured serum iron markers, fluorescent intensity of reticulocytes, and serum transferrin receptor (sTfR) concentrations in 374 females aged 14-19 years. Reticulocyte subpopulations were analyzed by flow cytometry, and sTfR concentration was measured by enzyme immunoassay. There were no significant differences in the values of reticulocyte subpopulations and RMI between ferritin alone-depleted group and healthy controls. However, middle- and high-fluorescence reticulocytes and RMI were significantly higher in both the serum iron- and serum ferritin-depleted groups than in the ferritin alone-depleted group. Middle-fluorescence reticulocytes and RMI increased gradually as the body iron store was depleted and were 3.4- and 3.6-fold higher, respectively, than normal controls, when the subjects attained a frank iron-deficiency anemia. There were no significant changes in the values of red blood cells or total reticulocyte counts during iron-depleted states. The mean value of sTfR (3.98 mg/l) in the subjects with RMI > or = 1.5% was significantly higher than that (2.26 mg/l) in the subjects with RMI < 1.5% (P < 0.01). The sTfR concentration correlated significantly with RMI (r = 0.61, P < 0.01) and middle-fluorescence reticulocytes (r = 0.59, P < 0.01). In short, body iron depletion induces elevation of immature reticulocyte fractions and RMI.