Clinical significance of immature reticulocyte fraction determined by automated reticulocyte counting.

The Sysmex R-3000 (TOA Medical Electronics, Kobe, Japan) evaluates maturation of reticulocytes by quantitating the fraction of reticulocytes within low-, middle-, and high-fluorescence intensity regions. We defined the immature reticulocyte fraction (IRF) as the sum of the fraction of high-fluorescence intensity regions plus the fraction of middle-fluorescence intensity regions. Then, we studied the clinical significance of IRF in the evaluation of anemia by comparing the IRF with the absolute reticulocyte count (ARC) and with the reticulocyte production index (RPI) and by reviewing pertinent clinical information about the patients. In the study, 132 specimens from 102 patients undergoing evaluation of anemia were analyzed. By using simple regression analysis, our results showed that the IRF has a weak but significantly positive correlation with ARC and with RPI, indicating that IRF is an additional useful parameter to evaluate the erythropoietic activity in anemia. Interpretation by integrating IRF and reticulocyte enumeration (ARC and RPI) provided useful information for further subclassification of anemia. Increased IRF (IRF > or = 0.23) and increased ARC generally indicated an adequate erythroid response to anemia. All but three specimens with an IRF less than 0.23 showed an RPI of 2 or less. These specimens were from patients with underlying diseases known to lead to decreased erythropoietic activity, predominantly chronic renal insufficiency. Specimens with a subnormal or normal ARC (with a corresponding RPI < or = 2) but with an IRF of more than 0.23 were from patients with various underlying conditions, including acute infection, iron deficiency anemia, human immunodeficiency virus infection, sickle disease with crisis, pregnancy, and myelodysplastic syndrome. Our results indicate that an IRF of 0.23 or less in patients with anemia reflects bone marrow that is nonresponsive or underresponsive to the anemia. Patients with an increased IRF (IRF > or = 0.23) may require further examination to clarify the cause of the anemia.