**8. Reticulocyte maturity index (RMI)**

Different populations of reticulocytes depend on the content of RNA in the enucleated cytoplasm. This RNA content can be assessed by the different intensity signals of fluorescence or light scattering/absorbance obtained on the flow cytometric based automated hematoanalyzers. The software-based algorithm in the autoanalyzers discriminate these reticulocytes into three areas of clusters according to stain intensity. High-fluorescence light scatter/absorbance reticulocytes correspond to young or immature reticulocytes, whereas maturing reticulocytes have medium-fluorescence light scatter, and older reticulocytes have low fluorescence light scatter. These three classes are called high-fluorescence ratio (HFR), medium high-fluorescence ratio (MFR), and low-fluorescence ratio (LFR), respectively.

The RMI is directly proportional to the amount of reticulocyte intracellular RNA. Maturational stages of reticulocytes in peripheral blood depends upon the level of anemia, disease state and iron status. Thalassemia, megaloblastic anemia, anemia of uremia and myelodysplastic syndrome (MDS) are associated with delayed reticulocyte maturation. Hence, RMI is used as adjunct parameter in the evaluation of hematological disorders and therapeutic monitoring of erythropoietic activity. Responsive marrow is expected to manifest a high RMI along with a subnormal RC.

### **9. Immature reticulocyte fraction (IRF)**

Immature reticulocyte fraction is better accepted term internationally to quantify the younger fraction of reticulocyte as a sum of HFR and MFR, to avoid the ambiguity in the interpretation of different fluorescence intensities of RMI. It is expressed as a fraction (0.00–1.00). IRF assesses reticulocyte maturation by the intensity of the staining that reflects the mRNA content. Assessment of IRF is clinically useful when it is evaluated in correlation with absolute RC.

#### **Clinical utility of IRF:**


### *Reticulocytes-Mother of Erythrocytes DOI: http://dx.doi.org/10.5772/intechopen.107125*


*L: Low, N: Normal, H: High, ARC: Absolute Reticulocyte Count, RMI: Reticulocyte Maturity Index, IRF: Immature Reticulocyte Fraction.*

#### **Table 1.**

*Diagnostic utility of reticulocyte indices [34].*

anemias. In ineffective erythropoiesis, IRF is increased while reticulocyte count is reduced or normal, in some cases of MDS or in dyserythropoietic anemia.

• **Screening of Hereditary Spherocytosis:** High RC without proportionate elevation in IRF can be suspicious for HS. A RC/IRF ratio higher than 7.7 can be used as a cut off for the screening of all HS cases as a diagnostic algorithm [33].

The clinical utility of IRF has also been reported in a variety of conditions, like: the monitoring of anemia treatment and neonatal transfusion needs; monitor response to EPO in a blood conservation program; renal transplant engraftment from Epo production; the detection of occult or compensated hemorrhages or hemolysis, and aplastic crisis in hemolytic anemias; and the early diagnosis and monitoring of aplastic anemias (**Table 1**).
